‘Crisis’ have plenty of volunteering opportunities available over Xmas and you don’t need any previous experience for many of their posts. They are mainly in London and therefore geographically might not be any use to you I’m guessing but I’m sure there are similar organisations in Exeter.
Homelessness is a huge problem that is found all over the country and many organisations could do with a bit of help at Xmas. Most organisations tend to operate shifts, with Crisis’ shift patterns being 8:00 to 16:00 then 16:00 to 23:00 and 23:000 to 08:00 and they require you to do a minimum of 3 shifts, so be prepared to fit in with the charities requirements. Other organisations may vary with shifts etc.
It’s very rewarding giving up your time to help others feel a bit better about themselves. Simply sitting with someone for a couple of hours can make a difference, even if you are just listening.
Crisis don’t require you to be CRB checked.