You're thinking about the £77pm the wrong way.
The psychological problem is that they have mapped the price of the battery to mileage, so it is impossible to rid yourself of the direct association between the price of fuel and the cost of the battery.
In reality, you are paying a lease of a lump of stuff, not for a fuel substitute.
Once you recognise that you are paying the lease for the cost of the battery, i.e. it is deducted from the purchase price, then it make more sense.
If you buy a new petrol car you don't think to yourself 'gee, if I pay £1,000 for leather option, that'll mean my fuel costs have gone up' so why do it with the price of the battery?
Don't get me wrong. The mental association between the battery rental and the fuel cost is overwhelming and I really struggle to keep the things apart in my own head, but just remember you're actually only paying for (and only getting!!!
) half a car without a battery, so it stands to reason that you might need to borrow a bit extra to pay for the rest.
If you were to look at the £77pm as a lease purchase of the battery, then that would be equivalent to borrowing £6000 for buying it, spread over 8 years, at 6% APR. People borrow that sort of money to buy cars all the time, and they never think to themselves 'gee, I've just increased my fuel costs by taking out a lease!'.