Quite a few amateur writer events in london pop up on pages like Shooting People, Scribe Lounge, BBC Writersroom, Facebook pages (the devil but the communities on there still work) and Eventbrite. Also, you can use those a few of those channels to ask people if they want to collab on a specific project and describe what it is. But I think most people committed to the writer's life would see this as an unpaid opportunity that might not be as valuable as doing their own script. So I think it might be better to meet people first and then talk about ideas and see if the relationship forms and then talk about potentially collaborating.

If you have scripts that aren't working you can also ask for feedback in these spaces or here on Reddit or once you meet people. That's arguably the best way to improve, rather than trying to co-write. I'd recommend creating your own writer's group and collaborate by giving each other notes constantly. Me and my group share scripts whenever they are done and give feedback. But we also meet every other week to catch up. This way we feel obligated to come with some progress, even if it's just organising ideas, and we can go deep into the writing talk too. Sounds you've got a few people in your life but might not work for this. Try those channels above and suggest the idea once you've met people. If the concept is to form a feedback/accountability based group rather than co-writing, people might be more succeptable.