I am a British citizen on L1 visa. My boyfriend is a US citizen and we intend to marry. If we seek the fiancé visa, will I have to go back to the UK for any time? And if I intend to relocate within the US to collocate with him (and therefore will need to find a new employer) what kind of working visa could I obtain?
You can just get married and apply for a green card directly while you are still in USA. You can continue working on L-1. In 90 days or so, you should also receive your EAD, if you apply.