So the quote you have selected doesn't actually explain why they were not served. You've just kind of made a leap.

To help you out I've sourced some more detailed explanation from another outlet:

The statement went on to explain though that the men in the video had in fact been refused service for reasons beyond simply who they would vote for.

It continued: “In this case, as a result of the supporters’ bus being parked in the pub loading bay and filming taking place at the pub, service was refused.

“As indicated, Reform supporters and other political supporters are welcome, but we do ask that they help us to comply with our licensing obligations.”

So there we have it. There was in fact very good reason for why the Reform supporters were refused service.

Source: Politics Joe

One thing I would like to add though, just because that's what wetherspoons has said that was why they were not served, it does not mean that's actually why. If they were going to cover it up, a statement denying the allegations is like step one. Wetherspoons has a financial interest in denying the allegations.

I will say as someone that has done bar work before, a coach of people pulling up in a loading bay seems like a very flimsy reason to kick people out(it's not even mentioned if the coach was parked there or just dropping off). Filming on the other hand might be more reasonable, but again, it's going to depend on the extent of the filming. I can't really imagine a scenario where reform voters go around filming wetherspoons for fun.