Sorry about your chook, but as a life long ornithologist I think it very unlikely to be a bird of prey. Though most British species would be quite capable of taking a hen, it is improbable that any would do so in the circumstances you describe (within a pen - therefore I assume quite a restricted space surrounded by mesh fence, between houses & in a village). Buzzard, Red Kite, and Peregrine have been mentioned - I would immediately rule out both Buzzard and Peregrine, as taking chickens from a village garden is just not their style. Buzzards prefer to hunt in open country and given the choice, would shy away from human habitation, besides, at this time of year food is plentiful and they are more likely to be 'worming' in the centre of a large field. Peregrines have a very specific style of hunting that would certainly not involve taking hens from gardens - again, their preferred food is plentiful at this time of year (wildfowl & waders including Lapwing / Golden Plover flocks). Red Kites are the largest raptor you are likely to encounter and could easily take a hen, AND would readily enter gardens to feed, BUT prefer carrion to live food and again, there's no shortage of this around at this time of year.
If I had to put my money on a bird of prey being the culprit, I would opt for Sparrowhawk. Gardens are certainly within their remit, and a large female hawk could easily kill a hen, but would possibly struggle to carry it off and would seek to dispatch it before doing so - leaving a lot of evidence in the way of feathers on the ground.
But to be honest, I think your silent killer is far more likely to have had four legs ....