DFC,
As a pilot I am very aware of the UK differences - and would not hesitate to file an MOR if I had a wake encounter that was possibly as a result of these local adjustments to the ICAO categories which operate not just on a national basis but in some cases are only applied at specific airports.
Are there differences between airports now?
Thankfully there is a European Directive landing on the desk of ATS standards that requires the UK ATS Authority to align it's procedures with ICAO as an interm measure prior to the introduction of European ATS Requirements. Let's hope a few of these "local differences" can be weeded out.
Ironically, the RECAT project to reclassify wake vortex categories has proposed a six category system that looks very, very similar to that which we in the UK already use!