I actually worked for an architecture firm in London, and the firm had sponsored my tier 2 general visa. Although, I'm not an architect- I am on the sales/ business development side. It's a long story how I got there, but I would suggest finding a global architecture firm in America, working for them for a year than transferring to their London office. It would be very, very tricky to get a local UK architecture firm to hire you.. reaching out to global firms will be your best bet. Good luck!