You could live anywhere you like as a short haul FO, you'll just need to have a room somewhere near LHR you can stay in. As a junior FO your roster will be at the mercy of the company. You might have lots of nightstops in row, you might just have day trips for a whole month. Or you could get a Reserve month when you spend 21 out of 28 days on 2hrs notice. The one thing that is consistent is that there is no consistency (apart from working every weekend). But there are plenty of people living further away than Belgium; Scotland for example. Not entirely sure about taxation but if you assume 25% tax on your first £22K and 40% tax on everything else plus 2% National Insurance you won't be too far off.