Thank you for the thoughtful responses.
As I said in my OP, there is no UK equivalent of KEH. The closest we have is LCE, who while they now primarily retail new digital equipment, also sell used film equipment. Their stockholding is however nowhere near the range and depth of KEH (although in the pre-digital world, they had a similar range and rep)
Unfortunately, prices in Japan don't reflect those in the UK. A Mamiya 6 in decent used condition - with a single standard or medium-wide lens - will cost around £1000 from a reputable dealer. A 500CM (in "user" condition) can be around the £550 mark.
I'd already looked at Juergen Kreckel's site, and it's a mine of information. He has a good rep too. VAT and Import Duty is chargeable on all goods over a certain value coming from outside the EU, whether from a private individual or not. At the same time, it's a bit of a lottery as to whether you get hit for it - I suppose it must depend on how stringent the Border agency are feeling on a particular day, or how scrupulous the carrier is about reading the customs declarations on each package. I know of someone who got charged when a camera he had sent to Fedka for repair was returned, as the charges are also able to be levied on services.
I suppose I will have to revisit my aversion to TLRs, but I simply dislike the way they look. It's important that a camera is something you want to pick up and feel happy holding. If I pick up a 500 and it felt good, then the size issue would disappear in a moment ...
The P6 option is a nice thought out of left field ... annoyingly, had I been in this spot about 18 months ago, I could have bought a pair of MF SLRs (a P6 and what I think was some sort of old Pentax MF) for a tenner at a local charity shop ... instead, I picked them up, smirked a bit at their quaintness and moved on ... and I hadn't come across the Bronica C, that looks a very interesting option indeed.
There's no rush. My picture-taking and general approach to photography is slowing down and becoming more thoughtful, and perhaps one day I'll even use a tripod ... so I can wait 'til the right thing turns up.
Finally, though, any thoughts on handholding the MF SLRs? (Sharpness is no longer my most significant goal when I take photos)