FL260 by MARGO in the prevailing SW wind really isn't that restrictive at all,and it generally gets cancelled during quiet hours which is 2200-0700 and at certain times at the weekend. Even when told no restriction most aircraft types still cross MARGO around and sometimes below FL260 anyway.
I'm sure 10W would probably be able to explain better than me, but almost all of the level restrictions in UK airspace are designed to maximise overall capacity, which is what the airlines want. However if the airlines put pressure on nats so that traffic could always fly at optimum cruise levels and have continuous descent from cruise to touchdown, with the aim of saving fuel,then i'm sure nats as the service provider would be happy to oblige. The downside of this would be a massive drop in capacity due to increased complexity.