The UK does not apply any departure separation for another "medium" (or "small") aircraft following a 757. The pilot can, of course, request increased separtation for any reason she/he likes. This is frequently the case at EDI for CRJs following 757s!
I remember hearing somewhere (and therefore cannot vouch for this) that the reason for this is historical; when the 757 first came to the UK in the mid-eighties, most were registered with a lower MTOW (99 tonnes?) for tax reasons (I believe), hence well within the UK "medium" vortex category. Elsewhere in the world, 757s are had a much higher MTOW (138 tonnes?) and were treated as heavies.
This would also explain why in the UK the 757 is thought of as a "high performance" aeroplane, and as a pig elsewhere in the world
