To be so upset and embarrassed

I know it's scant consolation, but the fact that you made the shortlist means that you were in the running; any one of the candidates met the criteria, and they had to pick one, and differentiating can turn on a whisker.
Ironically, my last school hired someone who proved spectacularly clueless, yet the head told me (the old sweat coming up for retirement, so no stake in the matter) that he was "head and shoulders above" the rest of the applicants. In the independent sector, it was possible to "nominally" appoint staff for just one year initially, and usually at the end of the year, confirming their continuing in post was purely a formality, but in this case, it was discreetly agreed that things were not working out.
I suspect, like other PP's, that they already had one candidate in mind. I had previously been in that situation, to my annoyance as I had come back from a seaside break (different half-terms) for the interview, and it was obvious to all us shor listers that we were there to tick the boxes.
I hope you don't feel taken for granted, but......how about gingering up management by asking for an afternoon off to look at another school where an interesting post has been advertised? Just an evil thought to cheer you until you feel better!

I’ve been there. Interviewed twice at a school I was doing supply at. I’d done loads of work for them while the HoD was off for 6 weeks and I was the only teacher in. I did a great interview, great feedback - they appointed an NQT and I’m UPS. Something will come up.

Also, can totally see typos and missing capital letters on my post! Please don’t judge my teaching abilities, I’m going to blame it on my phone!

Fellow secondary teacher here - teaching interviews are absolutely gruelling at any level with all the different aspects so can absolutely relate to your being off your game due to illness.
I went for a year head position in house a few years ago, didn’t get it and felt similar humiliation - I was the most experienced teacher there, the person who got it just did better on the day. I found it really tough to go in the next day, but within a couple of days had got my groove back and didn’t feel so raw.
if you like the look of this other job, go for it! You have already reflected on how your lesson went wrong - use this to ensure your next one is better! Nothing ventured, nothing gained and all that…
xx
p.s what subject are you? I’m English xx

You're not well. The interview is in the past now. Nobody will think badly if you didn't get it.

I’m sorry you feel so down about this.

I feel it was unfair that they didn’t let you finish.

FWIW I worked in a couple of jobs where multiple people went for head of department of similar more than once and still didn’t get it.
The only ones I know who have got those jobs were when they applied externally.

My old school constantly hired new people instead of promoting within and every time it made the internal applicants feel like crap.
They now have hardly any original staff and the new ones have either left or just aren’t that good.

Don’t take it personally OP.
It may have been as simple as not wanting to advertise for your role.
By you not getting it, your role continues to be filled and one of the other candidates can fill the new role.

Some days you're the pigeon and some days you're the statue ' commiserations OP.

Total sympathy. I went for in-house interview and messed it up rightly. They bow I can do the more senior job but they have to give it to the person who gave best interview. Went back in, head held high

Learn from the experience, you have already listed what you could have changed/improved

It sounds like that candidate was always going to get it. They probably planned to cut one out and it just happened to be you.

I also sympathise.

I went for an interview (in house) for a job I'd done before and totally ploughed it. I'd had good recommendations as Id worked with this team before and the interviewers were looking at each other with "WTF" expressions.

It's horrible isn't it. Sorry OP.

Don’t beat yourself up. Sometimes you get the promotion and sometimes you don’t. That’s life.

These things hurt at the time (been there!) but try to take the lesson in the experience. I know it’s easy to say that and harder to do it but you will look back on this moment and it won’t hurt so much. Just keep on going and trying and getting better and better

I sympathise. Teachers work really hard and don't get enough credit for it. These situations are really tough and you have to tell yourself that you did the best in the things you can control - preparation and doing your best during the lesson even though unwell. You can't control how others assess you so don't worry about it. Also good teachers do get feedback from happy students passed on to other staff- they see it from the way students smile when they are told Miss X or Mr Y is taking a lesson or session, or pupils might actually mention it.

Try to rest up, treat yourself to something pleasant bur healthy, and look after yourself, you deserve it. I'm sure you will feel better about this once you are recovered. If it's any consolation, good teachers are in short supply. If the outcome of the interview doesn't go your way, there will be similar openings at other schools where if successful you may feel more valued. Teachers move schools frequently nowadays- there are always schools wanting good teachers or experienced teachers and can offer better pay or happier working conditions.

Go for it. If nothing else, it is interview practice. Some people are better at interviews than others, so it is always useful to get into that zone. I am crap at them, so the more I can do, the better.

Besides, as the Romans used to say:
noli te nothi tere

(Don't let the bastards grind you down).

I was in this situation, applied for HOD at my own school and did get it. Retrospectively hugely relieved as new HOD is great and it’s a lot of work. If they know you and know you’re a good teacher I don’t the lesson holds as much weight- maybe there were other reasons. I think with HOD jobs if you’re keen you have to be prepared to move schools to do them. Sorry it was tricky but I wouldn’t take it so personally- it’s often useful to have an outsider with a new perspective.

If you don't feel valued at a school then it's probably not the best idea to apply for a head of department there.

I'd have a look around at other schools for jobs going instead.

You need to seperate your sense of being valued from this process. You have self assessed honestly and concluded that your performance didn't reflect your usual standards. You also understand that the assessment was based on your performance in the interview alone, and didn't take account of their prior knowledge of you, so really, there was no other viable outcome. I think its actually a kindness if an applicant falls short in an early stage not to put them through subsequent stages when the decision is not going to be favourable. It's hugely disappointing when we underperform when it matters most. But it happens to a lot of us at one point or another. It wouldn't have helped that you were obviously starting a virus. Try not to let it colour your positive view of yourself and your value.

That doesn't sound like they handled an internal interview process very sensitively at all.

I rage quit in a similar circumstance many moons ago but hopefully you'll keep your temper better than I did 😂