You do say in your original quote you had contacted a firm , the guy who fits it won’t get the £300 , I also take it your supplying the charger socket ( edit : seen in your post now you are )Without seeing the job from what you say it sounds with travel , install , testing and cerfication it’s probably
half a days work . It actually doesn’t sound excessive to me in all honesty if your getting a specialist firm in who have gone through the process to be registered and trained on ev charging .
Prices have gone through the roof for all trades these last few years , I’ve seen plasterers , carpet / floor fitters / kitchen fitters charging £250 a day and there is no need for ongoing training to comply with regulations and amendments to be sat every few years not to mention yearly assessments and calibration and purchase of testing equipment .
Rob mentioned £150 a day , an electrician can get £23 plus an hour on site now for jobs on site and that will be guaranteed 40-50 hours a week with no overheads or visits to wholesalers etc .