I can't answer in terms of Jet2, but at BA commands on the A320 at Gatwick are going very junior, currently about 4-5 years. However, it looks like that is going to drop further as there haven't even been enough bids to fill all the available Gatwick P1 slots this year. Heathrow is currently running at about 7-8 years for an A320 command. For long haul you're looking at at least double that, but probably even more. My last captain (I'm on the A320 at LHR) had been at BA for 18 years and expects to be able to get a 747 command by next year, 787 the year after that or four more years wait for the 777.
All that aside, at BA your take home pay isn't massively determined by which seat you're in. With allowances, a maximum pension contribution and a few other deductions I'm averaging a take home pay of £4,200 a month at the moment. That's as a first year FO. Many guys I know who are on long haul are making nearer £5,000 per month.
It's all a case of how your pay is structured rather than the headline basic salary.