There is no point in applying for a spouse visa for the US until, roughly, six months before you wish to leave.
If you are eligible to file the I-130 in the UK, through the DHS office at the US Embassy then you will find it much faster than if you are forced to file through one of the service centres in the US.
Either way, it takes much longer to get a US spousal visa than a UK one - be prepared.
Once approved the visa is valid for six months.