If you book MAN-LHR-somewhere , then the UK Dom portion takes on the nearest available fare code. D and I do exist on UK Dom, you just can't buy them direct, only as part of a multi-sector trip.
The last time they did one of these deals I went from EDI-LGW-NCE - outward UK flight was in D, the return was in I. The return UK Flight showed "Business UK" on the BP.
Under the new rules, and on this special offer, I can get an EDI-LGW-BCN in Club on the second sector for £277 and get 120 tier points (20+40+40+20). A full fare return UK Dom flight is £318 and I would get 40 points.