Hi,
I recently won a Perkeo I on Ebargum and I was out taking pictures with it last week around Shropshire, so you are not alone
I already owned a Zeiss Nettax, Nettar and Ikonta - all 6 X 6 folders, and have now added the Perkeo and an Agfa Isollette II. I am staying in Torquay tonight and have just enjoyed a walk along the seafront taking pictures with the Isolette on FP4+
I also live near Shrewsbury and can tell you that you can buy 120 roll film in Jessops either in Shrewsbury or Telford - although the range is pretty poor. The bigger branches of Boots in Shrewsbury or Telford (or any other biggish town) will stock HP5+ and XP2 and maybe a colour film or two in 120. Smaller branches only do 35mm. It is very easy to buy roll film on the net, though, as I'm sure you've found.
There is a shop in Walker Street in Wellington, called unremarkably 'The Camera Shop' - which still runs a mini-lab. The guy who runs it is a keen photographer and knows his stuff and in the past has processed 120 roll films for me. These will be colour, but XP2 is a colour technology Black and White film. On colour paper it usually gives sepia prints, which can look very nice. Alternatively you can shoot XP2 and send it off to Ilford, check their website. XP2 isn't a bad choice if you want black and white and intend to get the pics printed by someone. Less useful for home processing. For colour I think you can sometimes find Kodak Portra 160VC or 400VC in Boots / Jessops and Fuji Pro 160 or 400. You'd could send these to somewhere like Lab 35:
http://www.lab35imaging.com/120_medium_format.html
As for the viewfinder, very few folders (none of the ones I own) have any sort of distance correction or compensation so as already mentioned beheading is a possibility for close-ups, but these are perfect landscape (city scape, industrial scape etc) cameras. Keep the subject big and well away from the lens and it isn't a problem at all.