It isn't only motorways where the limit varies by vehicle type. Dual carriageways have similar limits to motorways. Single carriageways have more variation.
Non urban single carriageway defaults are 60 for cars and vans, 50 if towing or various categories of PSV and LGV, 40 (Scotland only now) for HGVs.
https://www.gov.uk/speed-limits
There is no technical reason why the default urban limit has to be the same for all vehicles.
And it isn't necessarily the case that cyclists think it
should apply to ever other road user than themselves, but it is fact that currently they do.
Whether it is worth changing the law for the very few cases where cyclists are traveling above the motor vehicle limit is debatable.
Whether it is worth also forcing cyclists to have visible ID such as registration plates is just a non starter IMO, but without it, cameras would not be able to enforce any speed limits that became applicable.