How to determine UK building value for US tax depreciation

[Ian Heritage]

I am submitting my US tax return for 2011 & I have a house in the UK which I am renting out. As part of my return I need to enter the % of the building is worth versus the land for deprecation purposes.
The person helping with my return suggestes 80% building versus 20% land which I assume is some standard % in the US?
Anyway I'm not sure how to dertermine this for a house in the UK, but looking at my mortgage valuation I believe its more like 42% building verses 58% land.
So I would assume I should really go with the mortgage valuation as its probably more realistic, but what are the tax implications here?
I don't know the correct terminology here, but I assume I will be able to claim less loss back against the amount of tax on my earnings, but when I come to sell the house, the cost basis of the house will be higher meaning I will pay less tax on any capital gains.
Would apprechiate any advice - especially from those who have submitted a return that inclued a UK rental property.
Thanks :-)