If you asked for reimbursement it should have been a reimbursement of the Jet2 tickets, not a refund of the U2 tickets.

But you can't double-dip. It's a reimbursement of the Jet2 tickets or a refund of the U2 tickets.

If you asked for- and received- a refund of the U2 tickets at the time then that's technically where U2's liability ends. If they've refunded your U2 ticket and given you the difference between the U2 fare and the Jet2 fare then that's the most you'd be entitled to- and is probably more than you'd be entitled to, tbh.