Everyone does Christmas differently. For some people, yes, it's a family only affair and for others it's a free for all. For some it's somewhere inbetween.
If they were in the first camp, they wouldn't have invited you.
Is it that regardless of how much they do actually want you there you would feel uncomfortable?
If it's really not an option to go elsewhere, I would suggest...
...postponing Christmas Day until you can spend it with your dp. My dad and stepmum do this if one of them are working Christmas.
...booking into a hotel. Lots of them have a programme of events over Christmas, carols round the tree on Christmas Eve, mingling with other guests over mulled wine etc.
...volunteering? I'm guessing the places might be filled by now.
...going for part of the day. Would you feel more comfortable going after lunch has happened? I used to love people coming over to play games on Christmas Day night.
...can you tag along with your dp? Go to the services or whatever he's doing, help out in a voluntary capacity.