... because, ermmm, it wasn't. S.8(1) Theft Act 1968...

Robbery.— (1) A person is guilty of robbery if he steals, and immediately before or at the time of doing so, and in order to do so, he uses force on any person or puts or seeks to put any person in fear of being then and there subjected to force.

There was no-one in the building when they *burgled* it. S.9 Theft Act 1968...

Burglary.— (1) A person is guilty of burglary if —

(a) he enters any building or part of a building as a trespasser and with intent to commit any such offence as is mentioned in subsection (2) below; or

(b) having entered any building or part of a building as a trespasser he steals or attempts to steal anything in the building or that part of it or inflicts or attempts to inflict on any person therein any grievous bodily harm.

(2) The offences referred to in subsection (1)(a) above are offences of stealing anything in the building or part of a building in question, of inflicting on any person therein any grievous bodily harm therein, and of doing unlawful damage to the building or anything therein.