I wouldn't recommend it but,
many moons ago, when working on the computers at LGW for Bcal, I had an urgent call to Rotterdam. I didn't have my passport with me but left LGW on a "staff airside pass" as ID. The immigration ("exiggration"?) guy gave me a hard time saying that I'd not be able to come back in again.
Finally got to RTM, local agent saw me thru passport control and back again, and I went back thru the UK passport control with my "airside pass", explaining the situation and got no hassle.
Probably wouldn't try it these days though ("9/11" and "June 2005"), what with the beard, the suntan, and all the "Gulf State" stamps in my passport
