Did you open the conversation with the £600 electrician telling them that you needed it done within 3 weeks?
It could be that they've sensed you're under pressure and like insurance that gets more expensive the closer you are to the inception date they are giving you the "surge" pricing?
I would have said that you'd be in this position - with 3 weeks to go - regardless of what charger was going in, so that aspect of it is somewhat irrelevant. If it were me I'd get a local electrician out ASAP to size up where the consumer unit is, where you want the charger, etc and get a quote based on that.