Catch 22; The airline is required to get the passenger to the airport booked or, with the passengers agreement, another place.
So pax offered a (Hobson's) choice; We can get you back to LTN in a fortnight or so or, if you agree, to LGW the day after tomorrow. Your only other option is a 'full' refund.
I sonehow doubt they seek explicit agreement and probably don't record it but the regulation envisions delivering a pax to another point.
I'd check with one of the companies doing 261 claims. They won't take a case they know they won't win.