It's probably in the awful, AWFUL BA lost property system by now. Good news is, you will probably get it back. Bad news is, you will probably have to wait for updates and maybe even make a trip back to T5 to collect it.
I left my iPad behind a few months ago, and as you can guess from my comments above, my experience with the Lost Property department was exceptionally bad. At one stage I threatened to call the police, because they had confirmed they had located my iPad, but then denied ever having done so, and were unable to produce it - this was after asking me to drive to the airport to collect it. All was sorted out eventually, but it took many hours, and multiple trips between different terminals.
I hope you have a better experience than I did, and I look forward to hearing how this is resolved. Best of luck!!!
Oh, and
to BA for being so inflexible and not bringing it to the aircraft for you, especially as it appears it was located in the CCR.