Frozen,
No doubt you will get a barracking from the anti Jet2'ers that lurk on here for you even considering moving from BA to "lowly" Jet2, however you certainly won't be the first or the last to make such a move.
Your take home pay will depend on a multitude of things, not only the number of hours you work, but how much you elect to drop into your pension. The basic Captains salary is now circa £106k, on top of that you will earn sector pay which has just had a large increase in value, and duty pay, pre deductions you'll be around £10k per month (ish)
The interim pay deal, is, as it says, interim, there will be another increase in April which is when our annual increase generally happens.
Working days off is definitely possible, there aren't many, if any in the winter as there are a surplus of crew, however the busy summer usually sees a requirement to work on days off if you choose to do so.