Welcome to FT! I hope you stick around
It isn't a 'special' rule, just the way the fares are constructed. It's to ensure the most availability possible on GVA-LHR-GVA, so that BA can still accept a GVA-JFK booking if GVA-LHR is selling out but LHR-JFK is still wide open. Welcome to the dark art of fare bases, fare rules and revenue management!
Most ex-EU tickets would be J-I-I-J, and indeed non-London UK-originating tickets as well, even though it isn't a separate cabin on the domestic flight.