I tend to agree with you there. The word 'disco' has always had the (perhaps now negative) "Saturday Night Fever" connotation, which was fine when that was popular as a genre, then establishments moved on from being 'discos' to being 'nightclubs' to try to shake this off.
You can offer a "mobile nightclub experience" but would you ever call yourself a mobile nightclub? The public tend to stick with "mobile disco", it's that term that they will usually search for when looking for your services (or mobile dj, or wedding dj / disco).
The forum will, I'm almost certain, move to www.mobiledjforum.co.uk. I think that's the better description for it.
For the directory, should I be looking at mobiledjdirectory.co.uk / . uk?
mobiledjdirectory.com is taken, with a UK based site, which seems to be connected with ukmobilediscos.co.uk as well. Doesn't seem too popular, though, with a total of 34 listings. Quite a nice site, actually, using WordPress and a decent directory theme. Might keep an eye on that!