yeah UK local government is confusing!
Local government in England - Wikipedia has a decent map but some places are more “divided” than others.
Leicester is a “real” city in the UK whereas places like Banbury & Bicester are “towns”
It looks like Leicester City is also a unitary authority in its own right so presume why is level 6 in OSM as opposed to some places that actually are a bit bigger (Coventry nearby is one) but only level 8.
I guess Bicester with just over 500 streets is a good “city” for CS purposes and certainly is “huge” at least in street count for a level 10 even if the physical size is relatively small and so adjacent much more rural level 10 parishes will have similar size but often single figure street counts.
I guess “ideally” then for consistency you include every level 10 as a thing in itself and, like London, nest it in bigger things as additional challenges. London or indeed anywhere “big” is never ideal when say you do a Station Road or High Street in one part of a “city” but have to do all of them to count but when in some cases you can easily get a small street just by running past the top of it without even actually going on it.