Like many others small gifts are generally acceptable, I think up to £30 in value, providing it is notified to your dept head.
Anything worth more than that has to be declared, (I say think as I can't remember having reason to declare anything) and has to be officially declared on a form which disappears somewhere within the HR system.
Under the previous owners the rule was a lot more draconian, with only "token" gifts allowed (the examples given were items like a pen, a diary, or a baseball cap - we had American owners).
There is a company wide rule though that any gifts sent in at Christmas must be passed on and go in a raffle for all staff with the proceeds going to a local charity, which is fair enough.