So BMW was actually a bit pessimistic on charge on arrival, rubbish bank holiday traffic helped though. Stop/start speed up/down traffic is actually better for an EV than a constant cruise I've found. As it can generate quite a bit more of electric back into the car.
Going home from Wales it said I would have 16% on arrival home, actually had 19%.
Decided to use (even with discount) over priced Ionity charger at Magor just off the M4 on the way up to Wales. Charged to 100% before leaving home, arrived at the charger on route with ~59%, car said it would be 56%. (so again a little under)
Still infrastructure and people issues though, of the 6 chargers:
2 = Looked like the cars had been parked overnight and no one there. (middle of an industrial estate on a bank holiday)
1 = Already being used
2 = Broken
1 = I used
Another lady arrived in a BYD, she could not get the first charger I used to work. (we both got an authentication error) - She tried the other spare, also an error. So she had to wait for me, luckily I only wanted 20% so I was gone in side 15 mins.
Even though BMW nav was pointed to a charger it recognised, I don't think it pre-conditioned the battery for charge. At most I got 120kWh out of it and it quickly dropped to ~70.
Far away from the 350 kWh limit of the charger and supposed 225 limit of the car.

Splash and dash of 20% charge cost me £7.50 and that was enough for the complete journey into Wales and then back home again. Technically I could probably bung 10% or even 5% in and it would have been enough.
The i4 M60 with it's dual motors AWD though is far from the most efficient EV out there, suspect something far more boring could easily do the run in a single charge.
Granny lead, as others have said, slow charger adaptor for a 3pin plug. I don't need the car once arriving in Wales, enjoying the country walks to the pub far to much for that. So a slow charge over multiple days is not an issue.
I'd even go as far as to limit the output to 6amps and be very slow and not stress the old house electrics. BMW being BMW are being tight now and don't provide one from new, first I4's did but mine did not come with one. It's £120-£150 for the cable and given this is the first time in nearly 6 months I've charged away from the house, not sure I can justify one at the moment.