If you've been on MN longer than 2mins, you should know to put all crucial information in to the OP.
Drip feeding over 8 pages means there will be lots of people that miss the things you've added because virtually everyone told you YABU.
Of course it is an very odd choice (of your dp) to make, to exclude his own mother from the ceremony. That goes without saying.
If he'd gone NC with his Mum, then completely different, but as he has continued to have a relationship with her, she IS still his Mum.
You saying you wouldn't minded if she was there just makes it a very strange decision to save £200 by choosing the room that can only have 3 witnesses. In the context of all the other things you are paying for (you mention a meal and a party and I guess you'll be having a new dress and potentially hair, make up, flowers, maybe transport, maybe a photographer?) it seems a very, very strange way to try and save £200. What about other close family ? Your Dad, Grandparents, cousins, any siblings he has, his cousins, maybe best mates?
As has already been suggested, people close to you would potentially have been happy to contribute towards their meals or something if they were being included on the wedding.
Or, if you HAD to have the tiny room, then each choosing one witness would at least be more 'fair' or reasonable.
The fact you've included 2 members of your family but his own mother isn't there makes you (both) unreasonable.