i think it depends on the check in agent? (im not talking about the MPC benifits here)
I never had a problem with luggage <25kg(20+5 buffer?) (regardless to status) and I even had once checking in a 20kg and a 10 with a little bit of scruffy looking and begging and having 0 status, of course.
Whilst a friend had to pack like 8kg out on a 28kg bag (hence 20kg strict)
both happened in LHR...
That's Y,btw.