Some great questions...
I have not compared my electric bill before and after, but it costs around 3p a mile on electricity. Some Green electricity suppliers have special EV tariffs which allow you to charge only when the rate is low (typically overnight) but I haven't switched to one of those yet.
If you go on a long journey you plan in advance when you are going to stop to charge, and always have a backup plan in case the charger you are aiming for is out of order - although you can check its status online before you get there.
Don't know about Tesco or Asda, but DH charged my car from 0 to 80% (we don't usually go over that as the last 20% is much slower) for £4.86 at a BP Pulse charger at a hotel. It took 20 minutes. My current car is 6 years old though so battery is smaller. A newer, larger battery car would take longer to charge on the road - but can go further without stopping.
Don't know how many charge points are near me as we only charge at home. There are issues with reliability but the infrastructure is getting better all the time. There is an app called Zapmap which shows where all the chargers are.
With some of the new electric SUVs on the market (e.g. VW iD4 GTX, Skoda Enyaq) they have a range of over 200 miles. Not sure how far it is from West Midlands to Cornwall, you'd probably want to stop once for lunch on the way as towing a heavy load will impact your range.
My current EV is 6 years old, DH is about to buy a much longer range EV as battery tech has improved a lot. But my current car still suits my needs perfectly, so I'm not worried about technology changing. The UK is investing heavily in EV infrastructure and the manufacturers in EV technology and I think they will be around for a long time.
Post Covid I have not done many 2h+ journeys. But pre-Covid I was travelling regularly for work on 3h journeys. Often had to stay overnight & chose a B&B which had an EV charger.