LuganoPirate – I had a similar experience at LHR recently, with the added ‘frightner” that 2 armed police were positioned behind me when I was being questioned. Pehaps they were trying to protect me from other posters!!
The bottom line here is that carrying cash, within limits IS NOT AN ILLEGAL ACT. However, various organizations have the right to ask why one is in carrying any amount. As NTarrant says, £3000 is a high amount, however, I would have thought that a quick look at your passport, showing the different entry stamps is a perfectly rational reason why you have the equivalent of £3000 in various currencies.
When ever I have been stopped and searched (which I have absolutely no problem with at all) I have always found people to be courteous and polite. Mind you with 2 M16’s behind you, it kind of stops you being too argumentative.