As i understand it
You need to do class 1 medical, ATPL theory and CPL all through the same licensing body.
So you would need to get class 1 at Gatwick and ATPL's sat in the UK.
CPL course can actually be done at any school which is on the CAA's list of approved schools.
And there are a few of them in the US with CAA exmainers out there as well.
The IR MEP etc are add on ratings so can be gained in any JAR FCL1 country.
MJ