You can find most of the answers to your questions here:
www.ppjn.com
Assuming that you will probably start work for a TP operator salaries vary from £14000 (eek!) with Emerald (
http://www.ppjn.com/temeraldairways.html) to £24,336 with flybe (
http://www.ppjn.com/tflybe.html). This is not an exhaustive examination of F/O salaries and doesn't include sector or duty pay. For examply, with flybe, I believe you can reasonably expct to earn a further £4000 on top of your basic pay. The business case doesn't work particularly well until you either get a jet job or command. You may be lucky enough to land a 737 F/O job in which case you might be earning £38,950 with BMI Baby (
http://www.ppjn.com/jbmibaby.html).
Hope this helps. Remember that the bank has (or should have) a responsibility not to lend you more than you can reasonably expect to repay. Good luck.