I think your presumption in incorrect: there's a general rule that the right to use a highway only extends to vehicles small enough to fit along it, so there's no need for a statutory width or height limit on narrow carriageways (but having one conveniently makes it an offence to exceed the limit).

For street works, there's a rule (http://www.legislation.gov.uk/id/ukpga/1991/22/section/66) that undertakers must not obstruct the street "to a greater extent or for a longer period than is reasonably necessary", which suggests the existence of a general right to cause reasonably necessary obstructions. In this case, blocking all motor vehicles is apparently considered reasonable, and I don't think that blocking cycles over some width is likely to be any worse.