Sorry, I'm sure the rule will be buried somewhere - standard conditions of carriage a copy of which is available on request blah blah blah. I don't think they need to "directly suggest" it in any case - by purchasing a return starting in SF, you are implying that you will fly to the UK first.

Its pretty much a golden rule - flying the sectors in order - the power of airlines to target particular markets with particular pricing strategies would unwind if they permit what you intend to do.

Sometimes people abandon tickets half way through and do this because a return A-B-A might be cheaper than a single A-B. This is more difficult to for airlines to stop (various threads on this, including abandoning trips at transfer points and ramifications).

Depending on the fare rules, they may offer to refund or let you change it to a single starting in the UK on payment of the difference.

Do feedback how you get on.... I suspect many posters here haven't tried such a maneuver for a long while so it will be interesting to see if you get any lattitude