London (as in England, Great Britain, United Kingdom, Europe) is a relatively tight word-of-mouth place where lots of opportunities never really get advertised.
Having said that, the biggest overall website would probably be
http://www.productionbase.co.uk/
To get an overview of the UK industry this is probably the most widely read publication
http://www.broadcastnow.co.uk/
There are some agencies which represent editors and can sometimes be worth speaking to:
http://www.tovs.co.uk/
http://www.indepost.co.uk/
http://www.sohoeditors.com/
TBH, there seems to be a reasonable amount of work around at the moment but PMs are very fussy about experience and mostly want recommendations or a great resume that they can check out rather than a reel, so it's a tricky place to break into, although not impossible.