Guys - You are confusing RVSM with MNPS airspace.
The above procedures are used in Oceanic/MNPS airspace only. If your in RVSM airspace, and unable to comply, you tell ATC your not compliment and negotiate a plan with them and/or leave RVSM descending below FL290.
And to clear up any confusion, a level bust in UK airspace is 300ft +/-