Even at £55 I think 750 miles is pretty cheap. If a petrol car would do 50 mpg you would have used 15 gallons at about £6.50 per gallon = £97.50 so much cheaper than petrol.
Also from what I have read avoid charging on the motorways as whilst they do have more, faster chargers they are more expensive and usually have queues to use them. Better to come off the motorway a couple of miles and use cheaper chargers that are usually available without waiting. Use the Zap Map app to plan better and find out live what chargers are available and the cost etc.