British General seems to be screwfix cheap gear and probably not very British! The 3ph isolator feeding my workshop is one of them, the spark who fitted it broke the first one nipping the screws up!
Quite a few these days are quite specific and don't inter change. They might look the same but the terminal offsets and lengths can be different. Just get another one the same brand, I don't think they're that bad to be honest.
Please be aware that changing from a B to C type mcb can cause issues with disconnection times. You'll probably be ok if there's a 30ma rcd in the garage D/B though.
But as mentioned above plug it into a different circuit to see if it still trips, try it on the main house ring as that will probably be on a 32A mcb and if it trips that the welder is probably faulty.
Oh, btw on the Ebay welder... received a message from the seller offering to show me a video of it working; not sure how I feel about that.
Don't you have to fit the same brand and not mix brands though... was a bit annoyed when the spark supplied the cheapest of cheap, despite me asking for a switched 16a socket and not receiving one.
I'm happy to replace the MCB for a superior brand as long as it is okay to mix brands in doing so...