I have a friend who had a hb in a flat (first floor) - with a pool. Can I ask why being a flat is an inhibiting factor with reference to using a pool?
I bought a pool to use in a planned hb in a flat (DC2), but due to being overdue didn't end up using it (induction in hospital instead - which was fine). I did use a pool for DC1, in a midwife-led unit, and it does give more actual back-coverage (I am quite tall) so was more effective than a bath. That said, a bath is still lovely, especially with a shower attachment that can be directed onto your lower back.
I did have gas and air with the first birth, didn't bother with the second. I only used a tens machine with the second, I'd say keep as mobile as possible, lean forward. I found breathing really helped too, if you are anywhere near London I'd highly recommend a breathing class I went to - it's only 2 hours, covers hypnobirthing breathing ideas but for only about £40.
I would say though that I also didn't go any further with pain relief as both labours were pretty quick - 5 and 1.5 hours respectively. If I'd been labouring for 24 hours+ or if they'd been back-to-back I think I may have seen things a little differently!
With the quick second labour I did realise I wouldn't have had much time in the pool if it took an hour to fill - it would also have been a distraction with DH faffing with it instead of supporting me (and would have possibly been an extra source of stress for him).
No idea if my ramblings are of any use. Hope it all goes well.