It could be a shrike, if it actually is a bird of prey, but is that small.
This is a woodchat shrike, which is found in Britain:
This is a brown shrike, which can be found in Britain, but which is more common in Africa:
This is a red shrike, and this was photographed near Cork, in Ireland--and this bird is also found in Britain:
Actually, i had a problem finding images of shrikes common in Britain. Most of the images i found were for loggerhead shrikes and northern shrikes, the only two species common in North America; and, for some reason, images of shrikes common to the Indian subcontinent.
Frankly, i'd be very surprised if the bird is still alive by the morning when you plan to take it to the vet. I advise that you consider very carefully how you will contain it overnight. Birds of any species are hag-ridden with parasites, inside and out. If the bird dies overnight while in your house, the parasites which are in it's plumage will leave the cooling body and look for another host in your house. It might be best if you can make it comfortable in box which you can then put in a shed, where you are certain it will be safe from predators.