I'm currently South London based, though not as far South as Surrey/Croydon, and may get back into the game at some point when we can be sure it's safe to gather...however, that looks like it could still be years! I'm pretty much in the same boat as you, in that I obsessed over the game when it first came out before eventually wandering off for a couple of decades. I'm more interested in playing again than I was this time last year, but whether I have enough time and energy for it in practice remains to be seen really. The related fiction is what I love most of all, and writing is a much cheaper hobby than table-top gaming, if a little on the anti-social side...
Model-wise, 54mm is definitely the way to go IMO, although you could potentially make the same characters in both that and 28mm if you really wanted to maximise possible games. 28mm isn't the way most people play but there's definitely a notable minority out there who prefer it, and if you're looking to "convert" 40K players, sometimes it helps if they can use their existing bits box to cobble together a cheap and cheerful warband for a test campaign. With that said, I'd strongly discourage letting anyone use any Space Marines if you do go that way, however convenient it may seem at the time!
Personally I love the larger scale so much, I pretty much stopped buying 28mm models in general after Inquisitor was released. £20+ per model does seem like a lot when you're buying them, and that's me thinking back to what they cost before GW discontinued the range, but I'd be surprised if even a really lavish 54mm warband from eBay cost half as much money or took anywhere near as much time to paint as the average 40K army. I'm sure it can be done on a shoestring if you've a keen eye for a bargain and a little bit of luck...or if you just ignore most of the OOP options and buy cheaper, more recently cast 54mm models from other companies, then whack on a few strategic purity seals.
