Well, I work in a Sixth Form, so we get people coming in with these qualifications.
As far as we're concerned, a BTEC First, if it's a Distinction, counts as 2 GCSEs. If it's a Merit, we count it as one. If it's a Pass, we count it as a grade D GCSE.
Having said that, we only count them if they've got Maths, English and preferably a Science as well. If it's a BTEC Science, it's not enough to go on to study A level Science.
What gets me is that schools tells their pupils it's worth 4 GCSEs, so we get students who were not allowed to take GCSEs who come in saying they have 16 GCSEs, when actually they have passed in BTEC First in Media, PE, Catering and Dance. Their bloody teachers just won't tell them the truth.