I simply remove my pump from my pocket and tell them what it is, depending on the security person one of 2 things happen, they either wave me past the body scanner, pat me down and then swab my pump ( this is the usual route ) or in the past when ive told them I can take it off they swab the pump off me and I walk through the scanner. I usually travel from Gatwick, earlier this year I went from Heathrow and they were a bit of a pain about my spare pump when I told them it couldn't go through the x-ray machine, it got sorted in the end and they simply swabbed it, its down to the security person you get and the one I had was foreign so probably just didn't really understand me! I'm always glad when I'm through but it has honestly never been a problem, just tell them what it is and show them.
The funniest thing was when I forgot to remove my sugar cubes in a small clear bag from my pocket, they obviously thought it was something else!! When I told the guard what it was and apologised she had a good laugh and said that she had never had that one before. Enjoy your trip.