I must say that you are the luckiest person I have ever met as U2 had no obligation to pick up the cost of the BA ticket. But, having agreed to do so, you could do one of two things:

First, you could initiate a chargeback for the original U2 ticket simply on the grounds that U2 cancelled and did not provide the service. I have no idea what the difference between the U2 and BA ticket amounted to, but if it was not much, perhaps that satisfies you in order to end things.

Second, you could initiate a claim against U2. You are not a UK resident (I presume this from your profile), so you cannot use the expedited and online MCOL process. But, you may still use a relatively simple process and I would note that courts use telephonic hearings and the like, so it is not as though, if U2 really insisted on a trial, that you would have to fly to the UK to handle your case.

I presume that if you file your case and attach the two emails and note that nonetheless, you have never been credited your funds, someone at U2 will choose not to throw more money at something which it presumably intended to end at the counter a good while ago.