Im in Australia and have a new T6.1 Multivan (2022). Not sure what you call them in the UK. No spare, instead they give you a tyre repair kit and pump etc. which I know will let me down when I tear a tyre. So spare is essential. Dont want to hang it on the tailgate, its electric and will stuff the motor I am sure. So its either inside or under. The wheel space under the car measures 72cm diameter (rough), I dont have all the bracket bits under the van either.
Was looking at going big wheel combo (19x8 with 235/55r19 AT tyres, like Yokahama Geolander G015), however the wheel diameter on them is over 74cm. so that is an inside the van only, option. So looks like I need to go slightly smaller (18x8 with 235/55r18) which comes in at 71.5cm diameter, so with the deflated trick, I should be able to fit underneath. Heaps more AT tyre options with an 18" wheel.