Thanks for the feedback. The money originally came from salary earned from work (plain old savings) and was deposited in a savings account that earned a little interest, then tranferred to a mutual fund that lost principal, then mixed with more job salary, then to a checking account that earned nothing, then mixed with more job salary. The checking account is joint with my Brit wife.
The exchange rate varied over several transfers between 1.40/£ to 1.55/£ in January to March 2009 time frame.
The account that it was remitted to is individual account owned by Brit wife, but will soon be converted to joint.
I guess it brings up another question: if I myself would be hard pressed to say what part of the 10K was principal vs interest, how would HMRC ever figure it out, assuming I was selected for an audit?