Combined degree in the UK for a US Student
In the UK, a bachelor's degree lasts three years, and the there are some undergraduate masters degrees available that last four years.

If you complete a year of study at a US university, some UK universities may accept this as enough experience to let you transfer into the second year of a UK degree.  This depends upon how closely aligned the two courses are, and how well you performed in the first year.  

The other option to consider is that many US degrees include a year of study in another country.