I have been asked to pay a basic travel Allowance of £1500 at Lagos Airport, Nigeria and told that I can get a refund at Heathrow - where do I do this?

There is no basic travel allowance... this is a fraud, and there is no place to get a refund at heathrow..

Who was asking for the BTA?

Let me guess, you have not really be asked to pay this? You are talking to someone who said they will come see you. and they told you that ?

A popular scam in under privileged countries.

Are you the one traveling or is this someone you met online who is asking for this?