That Oxford decision sounds fine. Only problem I have about it is that, if the same situation came up elsewhere, and the people involved tried to use it as a legal precedent, I suspect the court would be likely to see it merely as an exercise by Oxford of its discretion, which didn't have any binding authority.
But it might be worth people locally approaching their own local authorities and asking them to adopt the same policy. Like Nuclear Free Zones, if you remember them, but actually having a local effect.
What I'm not clear about the proposed abolition of PELs for licensed premises is, could that mean that rather than them being free to allow music, they might be even more limited than before? Unable to have music at all. I mean, it is an abolition of the need to get a PEL if you're a licensed premises, or is it an abolition of the right of a licensed premises to apply for a PEL?
I think a petition would be a good thing anyway, even if we're pushing on an open door - and it would be a way of making it more likely that our voices were heard, and make unintended slip-ups in the drafting less likely to happen. And it also might help draw the attention of more people to the existence of sessions. Even among people who go to festivals there are a lot who just aren't aware that there are such things.
I'd go for a simpler petition than Shambles - when it comes to actual legislation drafting inevitably becomes complicated, but I think a petition can be more straightforward:
How about this?:
We believe that people should have the right to sing and play music in the pubs of England, where the landlord agrees, and there should no longer be any laws that limit that right, over and above those that exist to prevent public nuisance because of excessive noise,and danger because of overcrowding.
We call upon Parliament to restore our rights in this matter.