This is where lots of BTL'ers are committing tax fraud: increasing BTL mortgages through MEW, but declaring all of the interest including MEW on their tax returns so it's offset against income. If you live off MEW, or spend it on cars, TV's etc then you cannot offset that interest against income on your self assessment return.
There are loads of BTL'ers doing this, this is where HMRC should investigate, perhaps starting with twonk Andy Shaw - who's been known to recommed this