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Who said anything about replacing your cars? There is no outright ICE ban, only a ban on new ICE vehicles. For a new car the price differential is around £5000 over an equivalent new ICE model, you'd make that difference up in fuel over the first few years of ownership, before accounting for really low depreciation currently.
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If you can afford to be an early adopter, probably. For a few more years, anyway.
The great majority of ICE fuel cost in the UK is tax. The only reason EVs are currently much cheaper to power than ICEVs is because EVs get an absolutely huge tax break for every metre they travel. That's only sustainable while EVs are a very small minority of vehicles. Once that stops being the case, that tax will be levied in some form. Hopefully on EV use, but I suspect it will remain politically expedient to pretend EVs are much cheaper to run and so the tax burden will be imposed somewhere else, including on people who don't use vehicles much or at all, forcing them to subsidise people who do.