Martial law.

[Ranger]

Here's a thought. Due to the limitations of manpower, why would they want to control entire cities. Unlike America, we do not have (comparatively), massive amounts of privately held firearms, legal or otherwise and especially ammo would be in short supply. Therefore, the general populace could not mount any kind of concerted attack on any well armed, disiplined and organised (para)military units. They could just defend strategic sites such as the City Of London, power stations, water/sewerage works ETC and let the plebs get on with rioting/destroying themselves and their areas. TPTB would probably bug out to rural retreats (Chequers?) with rings of steel around the major conurbations to keep the masses corralled until they burn themselves out. It wouldn't take long until the people were so weak from lack of resources that they'd welcome whatever forces came to their rescue and be utterly compliant with whatever they're told to do. I don't think they would try and lockdown the countryside, but there would be greater restrictions on movement and "gatherings", and, what with, checkpoints, patrols, observation points (OPs) and drones, movement could be severely curtailed especially if the OPs and drones are backed up with quick reaction forces. This to me, IMHO, would be a strategically more efficient use of limited manpower in a nationwide SHTF situation.
Just my two penneth.