Thanks all for the suggestions and advice - there seems to be such a wide variety of trainers and options that it’s a little overwhelming! Love the sound of an Aldi one (what a supermarket!) but unfortunately seems to be sold out 
With regards to power meters and dumb trainers vs smart trainers, do most people start with dumb trainers and still end up going on to smart trainers in the end?
If power meters sound like the thing to go for, what’s a good option for someone starting out?
You have 2 paths i’d suggest.
Dumb trainer at first with power meter
or
Smart trainer w/out power meter.
They’re equivalent cost-wise.
The first option though is way more versatile and more useful if you race and have a bike computer, and you can always upgrade to a smart trainer later,
where as the latter option is great if you just want to train on the trainer to get better and ride more recreationally while still seeing improvements in a more casual manner so that you don’t get dropped on your group rides and such.
Favero Assioma’s will be the least intrusive to your set-up, because they won’t require a new crank. They are the most accurate and reliable PM out there (for a bunch of reasons). They are also among the most reasonably priced. Miles ahead of the other pedal based PM’s and IMO, crank/hub based PM’s are a thing of the past.
Buy a set of those and a fluid trainer (wheel off or on, new or used, your choice) and you’ll be set indoor and outdoor. And you know your power numbers will be consistent and comparable throughout.
I had an Elite Rampa for a year before I got myself a Kickr. I found it to be a reliable piece of kit.
https://www.chainreactioncycles.com/sg/en/elite-rampa-turbo-trainer/rp-prod154153
If budget is a concern then I can definitely recommend a TacX Vortex. For £180 (Halfords/Chain Reaction) you’ll get a smart trainer that allows you to just ride, that’s at the same price as a fluid trainer and honestly the benefit and motivation you get from getting the full TR experience is well worth the difference in road feel. I have it’s baby brother the Flow as my second trainer and a Neo as my main trainer but split my time between two places) and while I can notice a difference I’m able to get perfectly productive training sessions on the cheaper turbo. Get a power meter when you have the budget in the future but for me a smart trainer is the way to go.