A UK locksmith normally charges three things: a call-out, labour by the hour with a one-hour minimum, and parts. Daytime call-out typically runs between £30 and £80 and labour between £50 and £100 an hour, roughly doubling overnight.
BS3621 is a British Standard for thief-resistant locks that can be locked by key from both sides. A lock meeting it carries a kitemark and the number stamped on the faceplate on the edge of the door.
Lock snapping is a method of breaking a standard euro cylinder in two to release the mechanism. An anti-snap cylinder has sacrificial sections that break off and leave the working part intact and still locked.
Measure from the centre of the fixing screw hole to each end of the cylinder separately, and quote the outside measurement first. A cylinder is described as two numbers, such as 40/50, because the halves are usually different lengths.
Yes, on the doors that matter, and it is cheaper than most people expect. You have no way of knowing how many keys exist or who holds them: previous owners, tenants, cleaners, builders, estate agents and neighbours.