I don't know the answer to your question, but I do know that if you have a Buy to Let Mortgage - only the rent is taken into account for your rented property at I think 85% of value. We have done that, and are about to build again, and have to borrow that way. We went to a a 1/2 day talk given by
http://www.themoneycentre.co.uk/workshops.asp? type=1&email=&link=ws0107 and found it really useful.. No pressure with anything, but I gleaned a lot of information, and it put in to perspective how to borrow on what you have.
The seminar was free, and there was a free lunch!!! I know I've always said there is no such thing as a free lunch, but in this case there was. If you can get to one of the seminars, I'm sure you will learn a lot - it was certainly an eye opener for us & I shall probably go to another next year.