The £900 will include: supply of a 7kW EVSE; adding a new circuit to your consumer unit/fuse box; running a few tens of meters of fairly expensive cable (6mm SWA suitable for outdoor and underground use) in a careful, tidy way; adding an extra ground point, if needed; an electrical safety test with a special tester; and issuing a certificate (which is mandatory for this type of installation). It'll take the electrician about a day.
People in the UK pay a fairly hefty premium as we don't have as many qualified installers as we probably ought to.
Of course, standard British sockets already supply 3kW - so the cheaper option is just to get an outdoor socket installed on an existing ring main. This is substantially quicker to install in most cases.