As Dave says, policies are down to individual bus companies. (If you need help finding out what your local company says, please let us know which one it is. However a simple phone call might get the same information even quicker).
For example First Bus says
"Folding bicycles are permitted on board our buses, provided that they are stowed safely and that they do not block the aisles or access to seats. While we will make every effort to carry folding bicycles, there are instances when this may not be possible, including:
(a) if there is insufficient space on the bus; or
(b) if your folding bicycle is likely to make the inside of the bus or its seats dirty, and/or could cause discomfort to other Customers"
Souce:
https://www.firstgroup.com/uploads/node_images/Bristol_COC_Sept2015.pdf
(That link relates to services in the Bristol area but I'm confident that First will have the same policy in other areas)
As a further example, Brighton & Hove buses have this policy:
"Folding bikes can be carried on our buses provided they can safely be stowed in the luggage area. There should be no protruding parts that may cause and obstruction"
Source:
http://www.buses.co.uk/page.shtml?pageid=1507