Hi, I don't have any direct experience but have worked with some current and former call handlers in other parts of the NHS so can give you some second hand feedback (and maybe bump you up a bit!). Most of the people I spoke to enjoyed the job and found it fulfilling although for the most part it wasn't something they wanted to do forever (lack of progression is an issue, there are supervisory and management roles available but a bit 'dead man's shoes' in that you have to wait for someone to leave to apply), they often moved on after a few years either to other parts of the NHS/emergency services (several ended up training as paramedics or nurses) or sometimes to a different kind of job altogether. They all said kind of similar things about the stress, that yes difficult/distressing calls did obviously happen but you are well supported and supervised and there were good procedures for what to do after you'd taken one, taking a break, speaking to someone, having a more formal debrief, access to counselling etc. Most of them actually found the most stressful/difficult part not the obvious dramatic 'talking through someone doing CPR' or 'handling a massive RTC' calls you see on telly which you get through on an adrenaline high, but actually the more low-level sh*t you get in any call centre role, the angry/horrible people who'd call up every day just to demand unreasonable things, complain or be abusive, or worse even than that, the really sad ones like the little old lady who doesn't want to bother anyone but she's been on the floor for 24 hours, lives alone and has no family, would someone mind coming to help her type thing. And bear in mind with the current state of the NHS, that little old lady could well be waiting hours and hours for a non emergency ambulance to help her up (along with many, many other patients who do also need help, just aren't top priority) which as the call handler you are pretty much powerless to do anything about but as the front end you will bear the brunt of people's natural anger and complaints. I do think you need a certain kind of person who is resilient and flexible enough to deal with all that then pivot to talking down a suicidal person then back to little old ladies who've fallen then telling off some silly kids doing a prank call then back to a CPR call all in a day, without getting overwhelmed, I guess only you know if you are that kind of person?
The call handlers I knew all spoke really positively about the team support and atmosphere, like a family is the phrase I heard used. There's usually quite good flexibility in terms of part-time working etc as well, and the pay while not great can often be topped up with overtime (in our area anyway, maybe it's not the same elsewhere). The ones that were more negative about their experiences it was down to a specific issue with management which can be the case in the whole NHS and I guess is a risk you have to take. One thing to consider on practicalities is that (obviously) it's a 24/7 role and the shift patterns can be quite brutal, that is not something that's easy to adjust to especially as an older person and also means your life feels like it revolves around work much more because e.g. if you are working Friday night and are in theory 'off' Saturday/Sunday you still really need the whole of Saturday and maybe into Sunday to recover and get yourself back to a normal sleep pattern by which time the weekend's gone... you may have more weekdays off in compensation but if all your friends/family work standard Mon-Fri you may find yourself missing out or frantically arranging shift swaps to try and attend important events etc. It is usually possible to apply to come off nights particularly if you have health issues but of course that's what everyone wants to do so there can be a bit of a queue to do so.
Good luck with whatever you decide!