MPC - just replied on another message board to your question re tax.
SooSoo - from my understanding, interest from mortgages can be offset against your income, regardless of where the finance comes from. e.g., if you decide to take some equity from your main UK property, then you can offset the additional interest that you're charged.
There is a great website regarding taxationweb.co.uk. Check out the forum, it covers all aspects of tax