At LHR my non-EU wife (although they don't know this as her passport has always been in her maiden name) has always been allowed through the EU channel with me; a number of UKBA agents have told me this is LHR policy. LCY is the same (albeit I sense that this is agent discretion rather than policy), but LTN, STN and SEN generally make us separate.

Fortunately, her home country returns the favour when we travel together, and allows me through as a local despite my UK passport.