Don't worry if the question has already been asked, this way is a lot quicker than trawling through the posts, and it flags it up again! Which is great. I have never gone through Stanstead but will be in 2017, so I am glad that you have asked this question.
You should have your letter from the DSN available, but I have only been asked for it once in 8 years. I do not even bother to tell them that my pens etc are in my hand luggage. They are so used to seeing them now that they don't even bother to search the bags.
I do tell them that I am wearing a pump and that it cannot go through an X-ray. It is fine in the metal detector, it is only a problem if you set that off and they then want you to go through the x ray thingy. Most of the time they say that's fine and just swab the pump. It is only at Bristol that I have had any issues, but that is rare.
I shall watch with interest to see what other responses y get.
Don't forget when you go away to take pens and needles as back up and take double what you think you need in cannulas, infusion sets, ...Ketostix, ... Accu Chek do a booklet with a list of what to pack, and I guess other pump companies will do the same.