there are LOADS of quality trance nights in London. Here are my picks...
Knowwhere - once monthly at the soundshaft. Quality promotion, nice people, top music. Ian Betts / Anthony Dean & PAblo + guests - highly recommended !!
Becomeone - once monthly afternoon party @ Sublogic bar. Only a small crowd of about 120 so a real house party atmosphere and very friendly. Music runs from Funky - progressive - "proper" trance - hard trance / hard dance. Nice way to spend a day
Ohm - small monthly party @ The Fringe Bar, Vauxhall. Trance trance and more trance + finishing on harder stuff. Next one this friday. Don't be put off by the venue, which is used for "alternate lifestyle" parties so is equipped upstairs with a playroom (very comfy having a bed to chill out on)
Logic is hard so not what people wanting fluffy trance will be after but it is a quality night
Pendragon is NRG / hard trance - top production, different sound to all the other clubs playing the same stuff