I believe they are typically only offered for 75% or less of the value of the place and generally only for fairly large amounts.
You could do quite well but you would need to take expert advice from a currency advisor. Interest rates are not the only issue to consider; you would also need to have a very careful look at the likelihood of the currency you borrow in strengthening or weakening relative to sterling.
Assuming you are paid in sterling, you could end up owing a LOT more than you initially borrowed if the loan currency strengthens against the pound. Imagine if you'd borrowed US$100,000 in sterling in the mid-90s when it was $1:66p (a GBP66,000 loan). Now sterling has risen relative to the dollar, you have to pay it off at $1:100p (i.e. your loan is now GBP100,000, even though your house in America is still US$100,000, so the amount you owe has silently gone up 40%) (I am pretty sure I have that the right way around, but don't quote me - I'm not an expert).
Unless you get a loan that allows you to switch the whole loan between various currencies without a penalty, you are making a 25-year bet on the relationship between two currencies, which personally I would not do unless I really, really knew what I was doing. If you are going to do this, I'd suggest finding a specialist broker who will look after the currency side of things for you, so you don't have to monitor the money markets yourself. Currency traders are paid to know what is likely to happen if XXX wins the Venezuelan election or if China de-pegs the renminbi, so leave it to them.