There are loads of dry ski slopes round the outskirts of London e.g. Becton Alp, Esher, High Wycombe, Aldershot, Hailsham, Guildford. A quick google hunt will give you lists of them e.g. http://www.natives.co.uk/whatsnew/dryski.htm
. These have links to their websites.
Do they have night skiing? Yes, in general, check for times. The toothbrush surface - Dendix - I've found much harder to ski on than real snow as there's generally a bit more friction and it's harder to turn. They aren't all toothbrushes - Hailsham claim to have a new type of surface (I haven't been) and Milton Keynes has real snow.
I've been to Aldershot & Esher dry slopes and there isn't really much difference. The main thing to look for is the length of the slope - they vary between about 90m and 150m.
Is it worth it? Probably not unless you very inexperienced. The slopes are shorter and gentler than most nursery slopes, so you will spend most of your time on the button lift. I'd say the 3 main reasons for going would be 1) to remember how skis feel just before going away first trip for a while 2) for beginners lessons 3) to get some time in new boots before taking them abroad.
If it's not too far, the Milton Keynes Xscape is much better than the dry slopes I've seen - it's about the same size as a nursery slope & with real snow is more like the real thing. However it's about £20 for an hour or so at peak times and I'm told does get busy particularly in school holidays.