I'm moving to London UK in January and want to know what visa and/or
work permit I should apply for and how

I own a consulting firm in the US, and my client is a US based company
with a London office.  We'll be consulting for them - investigating
the UK gift card market.  I'll need to be permanently based in London
for at least 12 months.  If need be I can leave for a weekend back to
the US and then return.

Thanks