Hi Jenny
I would definitely opt for mid week rather than weekend. Whilst there are obviously weekend bookings the majority of mine are weekdays (you know the married men fitting it in around work).
As for London location - that I cannot advise too much on as I am out near Heathrow. However, as Carla has said, you would definitely need to look at being in Zone 1 or 2, which are most central and the areas Carla mentioned seem to be the hot spots with most traffic.
Let me know if there is anything else you need to know or pm me and will help if I can. There are, I am sure, other girls on here who are more centrally based then me so hopefully someone else can offer more advice.
Alex x