Guys, I've never done it myself.
But I know someone who tried, and failed. Not at a border but by EasyJet as the visa was for France and the flight was for Spain.
If anyone is gonna take any risk then be aware that it's the carriers who wield and implement the power not border controls.
The rules state that the visa must be issued by EITHER the main destination OR the first point of entry.
Now, interestingly if the British Citizen is travelling with their spouse (who is UK resident), then technically even the schengen visa is not necessary to cross borders. However, you would need birth certificate and marriage certificate as supporting documents together with valid passport and UK visa.
The rub is that you have to actually enter a country in order to invoke the exercising of treaty rights.
Find a carrier who will transport you without a matching visa and destination and you're home and dry.
