I've seen a Deer taken bird seed to eat off one of the bird feeders on The Lodge live bird feeder webcam on the night camera in the dark over the last 3-4 evenings from YouTube. I've never heard of any of the different species of UK Deer taken bird seed from a bird feeder before. Maybe I'm wrong and others on this forum might have seen or heard of this happening before. Maybe a member of the RSPB staff can confirm that this does happen. I'll be most grateful if anyone can be of any help over this.
This is actually very common. Most species of deer around the world will feed from bird feeders if they find them and can reach them (or will feed on spilled seed below them if they can't reach). Both feeders aimed at 'garden' birds and those intended for feeding game birds will be utilised by deer.
Deer eating from bird feeders