Haven't we had this question before? And none of us had a clue what a BTA was? British Toilet Association or British Tinnitus Association, according to Google.........
Narlines, yes we have, read some of the answers, it's stands for basic travel allowance,if your fron the U.K. be a chap and give the home office a call and see if there is such a thing, or immigration we are being told they require this to leave the U.K. we all think it's a scammer tool to get money, but we cannot prove it
However, none of that has anything to do with actually being stopped by British immigration. If they're involved in scamming travellers in any way that would be a huge scandal; contact your MP.