Motorway charging points aren't free. They're £6 a go. An EV is not a cheap way of doing motorway journeys. It's either the same, or possibly more expensive than an efficient ICE.
PS - I don't know if you have to pay for the Tesla superchargers. But if you're thinking of buying a Tesla, you're not buying a car to save money.
I meant free as in available rather than cost wise. TBH though even at £6 a shot I don't think they are bad value. £6 gets me less than 60 miles in my Focus TDCi, probably nearer 50 miles at motorway prices.