433, from a London ATC point of view, not many use descent rates as you suggest, but only 'expedite'. Very few on even rarer occasions use rates specific to maintain separation, I think because it isn't ever taught, so we try other methods.
Therefore to me your statement suggests:-
1. Leave for FL260 now.
2. Immediately increase to at least 2000ft/min descent rate.
3. If unable to comply within a reasonable time (ie1000ft), inform ATC and possibly state maximum rate at current level, and FL where 2000ft+ can be achieved.
That what you were after?