There are tons of bridges around the UK with a 6ft 6 width restriction, for example Chelsea Bridge in London. The X is 6ft 6.6 inches wide.
However if you stand at the Albert Bridge (which is the one you meant with that width restrictions) you'll see tonnes of LR and RR going through which are wider than that as well as Sprinter vans which are about the same as the X... I can only assume the police take the view that if you fit through the width restriction (which must have some leeway on them as bigger LRs are about 81") they won't bother you...?