From what I've read, many of the UK utility companies will subsidise installation of an EVSE, so you will want to look into that. If you're done taking a long trip, it will take many hours to recharge your batteries...better in the UK with 220vac than in the US with our 120 on the plug-in unit, though. In the USA, charging units all come with a tethered cable...that's not necessarily the case where you live, at least if you were trying a level 2 charge. I think all of the CCS units have a tethered cable, though. If you do not have a cable for level 2 charging, you may want to buy one. For the narrow back roads and commuting, I think the i3 is a great car. You should have fun with it. It's okay for longer trips, but not my ideal. I prefer to just get in, and stop when I feel like it, not be required to do it as regularly as the i3 does on a longer trip.
The CCS unit availability in the UK I've heard is pretty good. I know in the USA, there are apps that will show you where they are, and the built-in maps of the i3 will as well, but you do need to update the maps to have the latest information. There are probably apps for your phone or tablet that will show charging points. In some markets, map updates are free, but it's easy to do and you can buy a license for free updates if yours does not have it that doesn't cost much.
Keep in mind that while people do successfully take long trips with their i3 REx, that is not what the car was designed for, and you're going to be stopping about every 1-2 hours or so to either fill up the fuel tank, or charge the vehicle when on a motorway after the first bit running the battery down. The i3's batteries can charge at a maximum of 50Kw/hr on a CCS unit, but it slows down as the charge gets higher, and will also be slower when the batteries are warm and more if they're hot. To fully recharge you 94Kwhr battery will take closer to 2-hours. Some of them seem to charge for both the power used, and factor in time. While mine has that capability, I've never found the need to do a DC fast charge.
Keep in mind that if you let the batteries get depleted, when you turn on the REx, or it starts on its own, it will not try to fully recharge them, only try to maintain whatever level you had (or try to get to 6%, which is where it will automatically turn on). The REx is about a 34Hp motor...the i3's motor is 170Hp...if your batteries are low, the REx cannot keep up on the power demand, and your state of charge will drop. It's fine for maintaining about 70-mph on the level, but faster, or with a grade involved, it will be pulling more power from the batteries than the REx can replenish until you drop your load.
2014 i3 BEV, 2021 X5 45e
(The i3 will be sold soon, <17K-miles, interested?)