For a EV the mileage claim rate is 8p per mile. How to I change the rate calculated in the mileage page on Quick books to suit the electric vehicle rate?

This has nothing to do with HMRC

HMRC set the final rates for both company car and personal car users

Personal users are set for reimbursement at 45 pence per mile for the first 10000 miles in the financial year and 25 pence per mile thereafter.

What the OP is asking is how those rates get changed in QBO so that the monies owed to him, by his company, are correct. that way he doesnt get investigated for tax evasion!

If all QBO does is set those rates to 45 pence then we have another headache to work around to recalculate everything. its NOT simple!

Company car users are different still. 

These rates MUST be calculated by the end users e.g. we, the quickbooks users. HMRC dont know how many miles we do they are interested in how much tax we are giving them or avoiding paying (in some cases).

There are apps out there that integrate into QBO but at £15.99 per user per month then you may as well have a mask on and shout "stand and deliver" as that is extortionate. There are other apps that do it for <£2.00 pm but they dont integrate to QBO