I have family there and am really fond of Devizes (much more so than Marlborough). If your budget can stretch I would take a look at Salisbury (grammar schools and culturally much livelier), but if a market town is what you want it’s nice.
Holy Trinity at Great Cheverall is a fabulous primary, and you can get in there from living in Devizes. In the town St Joes is now good again (having had a terrible off patch). For senior schools, Devizes isn’t great, but St Laurence’s (C of E) in Bradford on Avon is, St Augustus (RC) in Trowbridge is as well - but is said to be a bit off very lately. I don’t know if you have to be a church goer to get in to St L’s from Devizes but check that out.. You can technically get into the Salisbury Grammars with a bit of luck (I know someone who did) but it’s a hike.
As a PP said, the Wick Lane side of town will offer you the best chance of an annexe and is very nice (near the woods etc).
Nearest general hospitals w A&E’s are Swindon, Bath and Salisbury, which are mostly decent, with Bristol Southmeads for the big stuff. There is a health centre that does small procedures though. The Lansdowne and Southbroom GP surgeries are good.
You will use a car a lot. The bus links to other towns are awful. Pre Covid there were plans to build a station (an ext to the Westbury line I think) which have been scrapped. It’s a great loss but I guess it will happen eventually and at that point property prices will shoot up. Chippenham has the most London trains, and there is Pewsy and Westbury also. Bus links to the stations are bad and have got worse in the last decade.
Shopping and Theatre are Bristol and Bath. The supermarkets in Devizes are quite small, but you can get everything really. Countryside is amazing. It’s low on restaurants - lost quite a few in the last few years.
Coming from London the population is pretty old. There will be lots of U3A type activity for your mum. It’s a nice town for kids I think. They will run off at speed at 18, but there’s enough bits and bobs to do for 12 to 18s. You might find it a bit dull - I love to visit but not sure I could live there. The PP who says it has hippy Frome/Stroud vibe is really pushing it - it does not. I would look at Salisbury and Bristol/Bath for more going on before you lock it for that reason - but otherwise I am sure it will gradually get a bit livelier.