Just bash some number in a spreadsheet to suit your assumptions and it'd be easy enough to work out what it cost to run, a new fix would be approx 40/7.5 at Octupus, after October who knows but it will likely be higher.
check fuel cost for 7.5p vs 18/40/55 etc. to see what it will set you back.
As an example my sheet for mpg cost for elec vs petrol/diesel is here, take a copy and adjust to suit you, this will no doubt open me up to scrutiny from the forum, just enter numbers in yellow boxes for vehicles your are considering, this was just to calculate commute costs hence only 11 months etc, so you might want to edit that, things I do personally can cost what they like, I am happy to burn up a number of tanks in a day for fun, the commute to work to me is nothing but a burden.
You can just tweak the numbers for days/wk and miles such that the Mileage for year comes out at what you would do, that would be sufficient.
Things like tyres and servicing costs are basically an average I have historically spent over a few years reduced to an approx cost per year.
This one drive link may or may not work.
1drv.ms
To this add your electric usage for day multiplied by the different tariff amounts then you'll have ballpark numbers. for what you may or may not save.