Greg,
Summary from the UK Manual of Air Traffic Control:
The 250kt limit does not include:
1. Flights in Class A or B
2. IFR flights in Class C
3. Flights in Class C and D when authorised by ATC
<Some other stuff>
Importantly:
1. ATC cannot relax the 250kt limit for traffic leaving a "known traffic enivronment" such as Class A and entering F or G where primary separation method is see-and-be-seen.
2. In E, F & G ATC cannot relax the 250kt rule.
Hope this is of use.
Cheers
Foo