I was stopped at LCY one evening, a good few years back now. I asked the fellow why I'd been stopped, was it a spot check. The reply came back "we don't do spot checks, sir"

As noted above, if they're looking for you they'll find you!

Just to illustrate what happens elsewhere in the UK (and indeed used to be the case at LHR) passengers from Ireland are taken straight to the main customs hall via special corridors/buses etc. You then collect bags and go through the appropriate channel (which in the OPs case would be the green or red one, not the blue one...)