There are always so many applicants for CS roles, especially at the AO/EO level (or whatever they're called now), so you did really well to get as far as you did.
There's some really good advice about using the STAR technique and using this as a learning experience. A friend of mine 20 years ago had the theory that of every 3-4 jobs you applied for, if you were lucky, you'd get 1 interview, and of every 3-4 interviews, if you were lucky, you might get a job offer.
I also worked in a quango for a bit in my 20s with a lot of TUPEd civil servants, some of whom had worked in recruitment, and one year the sift for a 1 fixed-term 12 month AO role initially left them with 800 candidates, so really, you're doing well just to get to interview.
I was a specialist civil servant for 4 years before I moved into a similar job in the NHS, and I wouldn't pass the CS interviews now, in fact I'm not even sure I'd pass the sift, having seen what they ask on some of the application forms! I only got into the civil service because I had that specialist background and none of the other 4 candidates wanted the job though I didn't find that out until much later, luckily for the department!!
Also, friends who are still in the civil service (some of whom joined from school) will say that some departments are less difficult to get into than others as they don't get quite as many applicants, but that varies round the country.
Good luck, you'll get there.