Your best source of information on London:
https://www.punternet.com
TER is a good source for both places. You can access foreign reviews from the US if you use a VPN to show you are surging from abroad. At least it worked the last time I tried it.
Seekingarrangement.com has worked for me in Paris if you have enough lead time and are going to be there long enough to cultivate an arrangement. Bon chance!