I have never heard of such a case. Even if a volunteer did try to argue that being invited to a staff Christmas party constitutes consideration and therefore creates a contract, I cannot believe that any tribunal would accept the argument.
What a volunteer might possibly have done is include this in a list of all the reasons why the tribunal should find that there is a contract and therefore s/he should be entitled to employment and employment-related rights.
For details of cases that illustrate when volunteers might or might not be held to be working under a contract, see The Russell-Cooke Voluntary Sector Legal Handbook (Directory of Social Change, 2009), s.39.4.
You could also call it the Team Christmas Party.
Terminology is more important than we might realise and it is relevant to more than the Christmas party (which it could also be called). Many organizations have staff away days some invite volunteers along with not much talk up. Simply by rebranding it to Staff and Volunteers away day-volunteers will obviously feel fully invited and that their input is being sought. Which should help your volunteers feel valued and maybe even improve retention. 🙂