For most customers, the on board charging in the i3 is limited to 7400W, so divide that by your supply voltage would give you the maximum amps. Generally, a 30-32A EVSE is the maximum that the i3 can handle, but because of the protocol, a larger or smaller one will still work...the EVSE announces to the car the maximum that it can provide, and then the car will adjust its draw to never exceed what it was told by the EVSE. IF you choose a smaller EVSE, it will still charge the i3, but it will just take longer.
Most EVSEs are rated for exterior installation, so it could be either outisde or inside. I don't know the rules in the UK, but in the USA, a 30-32A EVSE would require a 40A circuit since the codes require you to derate the circuit capacity to 80% when the device could be on more than a few hours continuously.
In the USA, the charging cable is generally tethered to the EVSE...that isn't necessarily the case in the UK. So, that depends on which EVSE you end up buying as to whether you'll need one. I don't know what the public EVSEs have...you might need a cable for one of those.
Depending on the market, and the model year, the vehicle may only have the ability to recharge at half that 7400W maximum. Also, not all vehicles sold will have a DC charging capability. You can tell if yours does by seeing if the charging port has two sections with a 2-pin one below the main upper one. If you do, you can charge at public DC fast charging locations. Generally, you'll end up with about an 80% charge in about 30-40 minutes.
2011 535i x-drive GT, 2014 i3 BEV