I’m not a tradesman so not totally sure, but the CT clamps need a data cable from charger to meter to monitor load- the £35 version has a set length but many installers use EV ultra cat5 which is basically the power cable combined with data cable. It’s about £4-5/m more expensive than standard power cable and easier and neater than snaking a data wire around the power cable + £10-20 to fit the clamps and wiring
+ They likely have a standard mark-up and profit for supply items to cover occasional failures, collection or postage etc..
So if that applies probably not too bad. £700 seems about average for the smaller chargers, some seem to get incredible deals but perhaps their installs are REALLY simple.