Well I'm not. Some people may make that sort of cash at LHR, but it all depends on:
1) Basic pay (old A scale, DEP B scale or CEP B scale, there could easily be £20K difference in basic alone between these). Plus you may be a trainer so there's another increment.
2) Allowances. These depend on how good your roster is, more lucrative trips means a bigger pay packet, but they can be incredibly variable. A very good month might be £1500 to an FO, but a bad month could be £400. Or nothing if you've been on leave, reserve, sick. The flip side is the more time you spend away from home to get those allowances, the bigger your Diners Club bill at the end of the month, so you can knock several hundred quid off those figures.
3) Seniority. This is broadly linked to your basic pay scale, and dictates your ability to bid for the trips that will get you the most allowances, if that is how you choose to bid. It also allows you to achieve more time off if you want to bid for it, so that you will be more often be available for.....
4) Overtime. Nice if you can get it, but I'm usually far too busy.
One of the main problems with pay in BA is that due to the anachronistic system of pay and various allowances there can be enormous variations in pay on some fleets within just a few seniority points. There are plenty of senior long haul FOs earning more than junior short haul captains, and in some cases junior long haul captains. On top of that the pay system distorts the bidding process as people are compelled to bid for what will earn them enough cash rather than what they actually want to do, which means senior guys endless moaning about how bad NRT is (whilst raking in the cash) whilst the junior guys are stuck in Lagos for the fifth time that month on £2.80 per day.