A little history however - the UK has it's own wake vortex categories above and beyond ICAO standards. The UK, splits ICAO's Medium into Upper and Lower Medium. Also, certain UK airports such as LL can use 'reduced wake vortex seperation minima. This allows 2.5nm seperation rather than 3nm under certain conditions.
this document references all of the UK weight cats and wake sep distances as my other reference...
http://www.skybrary.aero/bookshelf/books/1166.pdf