Join BA, get a taste of LH and if you don't like it, it's an easy move to the 320.
If you don't like BA's fairly unique version of SH and want the luxury of a regional base with a 15 minute commute, take the rating back to Jet2. You'd very possibly have 320 command time and you've lost nothing vs staying at Jet2 for the command. Training etc all still available to you.
If you stay at Jet2 and wish you'd given LH a go in 10 years time, it becomes a completely different decision to give up command and put yourself through being junior at BA at that stage of life, plus you've lost 10 years of seniority if you decide to stay.
There's a multitude of reasons to try LH, but if you think you might still be wondering about it 20 years from now, then you may as well crack on and give it a go. This industry can throw all sorts up but at the moment I can't see any barriers to you heading back the other way if you want to.