yes, it's fine!! UK border control don't care what you take out. They care about what you bring in. Bury it amongst the tent pegs when you return to the UK.
Last time I went they searched my bags and found my knife. But, I said I was camping and they let me keep it and just told me to stash it at the bottom of my pannier
Anyone taken a knife/leatherman on a UK to Europe ferry?