Hi,

The transport for London website has a lot of details for London, England.

They used to say folding bikes on the underground must be under 20 inch diameter. I don't see that now. Interestingly the only distinction is folded or not. Nothing I could see on size.

Currently
Buses- at driver's discretion.
Trains- folded allowed at all times. Non folded not allowed during morning and evening rush hour.

More details on the website.