Tomotomp - if the guy said it was in Surrey, it was in Surrey. Not Bishopsgate. Notice how we use capitals in English.
XX621: you say you're not good with types, but you could see it was a twin. However you couldn't see a colour or reg. Hmmmm.
250 ft height is not illegal - so long as the heli remains 500ft AWAY from any person, vehicle, vessel or structure.
As you have not been precise with your location it's difficult to help - but "Surrey" would seem to imply that the aircraft was under FIS from Farnborough. Even if not, it would show as a blip on radar - although probably not if at 250ft.
So - best that you make an enquiry that way. I'm sure the Farnborough team have got plenty of time and manpower available to assist in your fishing.
Just seen your recent post
However, even if it was at 500' I'm still curious if private heli's can get special VFR in the Heathrow zone on a regular basis??
Er - yes. 7 days a week with exemplary service from AlanM and his screenwatchers.