all they have done is sent a tech out to connect it at a cab.
They shouldn't charge you the full whack.
Call them and ask what they had to do for the "install" and how it can cost that amount.
If they don't offer a discount advise them you are going to Ofcom and trading standards 
Boils down to what the installer actaully found existing. If they had to change either E or D side, I believe that now cops the full cost
As an ex switch engineer for a certain cable company it is crazy to charge that amount for an e-side d-side change its a 2 minute job on the programming of the switch (if that) and a 2 minute job in the cab but it does depend on the rest of the wiring i suppose
2 minute job at the cab? Oh no it ain't if you do it properly - gas test before opening, replace all dessicant packs, tidy up your vertical, before running the jumpers. Obviously, there is also travel time etc.
Also, remember it costs tens of thousands to replace (or more to install) an e side or d side cable, so if a new line uses a new pair in either, some of that cost has to be recouped.
And not all bt exchanges are pre-jumpered afaik (very few were in my day)