And why you want to do it.
You couldn't do an additional full BTEC Extended Diploma (that would be 3 A levels plus another 3 A levels) but a BTEC National Certificate worth one A level might be possible. But remember, you only need 3 A level grades for any UK Uni. If taking more than that risks all your grades, it isnt worth it. AAA will always look better than ABB+M - and that could cost you a Uni place.