The UK Safety Regulation Group has decreed that E135 and E145 aircraft must have the non-RVSM 2000 ft vertical separation applied within UK airspace, regardless of claims of compliance by the crew/company/manufacturer. Apparently monitoring has revealed a greater-than-acceptable variation of level in cruising flight (likewise the Avro RJ / 146 series). Whilst RVSM capability is not yet legally required above FL280, even in UK airspace, UK ATC have the option to deny non-RVSM aircraft access to such airspace for traffic reasons. That, in effect, means when using 2000 ft separation would result in a reduction of sector capacity (by effectively sterilising the levels 1000 ft above and below the non-RVSM traffic).
Hope that explanation clarifies things.
Edited to remove confusing brackets (!!!)
[ 21 October 2001: Message edited by: Numpo-Nigit ]