It's unlikely that you'll get a UK mortgage on a property in the USA. A mortgage relies on the property being used as security for the loan. The firm lending you the money needs to know that, if the repayments aren't kept up, they can take possession of the property. UK lenders are unlikely to want to get involved with the US legal system if things go wrong.
This doesn't mean that you can't get a mortgage. It would have to be from a US lender but there are UK companies who act as agents. For example, see here:
http://www.mortgages.co.uk/holiday-homes/USA/u sa-mortgage.html
Chris