According to Wikipedia, the standard size parking space in the UK is 2.4m by 4.8m, so if you want to fit in one you'll need a SWB - but you may well find that's too short for your needs.
I'd go for a MWB and accept that it'd be more restricted in parking - bear in mind that you won't be able to park in multi-story car parks anyway due to the height. Go for a medium height unless you're over 6' tall - you won't be able to stand in a low-roof one
One good suggestion I've seen is to mark out the space on the ground (interior sizes should be easy fo find for all variants) with tape or string and see if you can fit in everything. Also watch loads of videos of what other people have done - there's some amazing space saving ideas out there...
Personally, when I was considering the same I was advised by several people to avoid Transits - they rust, a lot. Most people I spoke to recommended the Fiat/Peugeot/Citroen 'EuroVan', which is commonly used for professional conversions.
Of course, being different, I've decided to use a Land Rover Defender 110 instead, when I can find one that I can afford...