WH Smith airside in T5, does not sell any sort of glue. They do sell Parker Quink ink catridges so there is hope for us all yet. This is the branch by Crown Rivers, the smaller one by Paul Smith always seems to be closed when I'm in T5. The T5B branch is bigger than either, but that was a research project too far. However I have an easy FT answer, namely a B2B to NCL where super-glue (£3), and a strangely comprehensive selection of laptop screen cleaners are readily available along with 80 TPs.
I must admit I thought it would be a banned substance airside, but I've checked the paperwork and it's not.