The average pay is currently slightly more than the figure you quoted in your post - add another £200-300, though this figure varies according to your roster and what you spend downroute.
You need to work out how much money you are likely to spend in an average month commuting to and from LHR.
You will be able to bid for the type of work you want (trips or home every night) but you must bear in mind it is a PREFERENCE system ... if you bid properly, and with some luck, you should get most of what you bid for.
Be realistic and prepared to do some of the unpopular work (very early starts, LHR-PRG-LHR, LHR-BUD-LHR) and at least one block of standby per month (typically 4 days which may include at least 1 or 2 Airport Standbys).
Currently FTCs tend to get more longhaul work due to the requirement of at least 1 FTC on every longhaul flight (not required on shorthaul).
Good luck, hope this helps.