I'm not fully aware of what you want in a van, if it's exactly the same as the one's you mention then why not buy a second hand one of those and be done with it?? I'm guessing it's the price. Well if you have the time to wait and don't mind the hassle of self build then fair enough, hope you are happy with the end result, I know we wouldn't be so that's why it's not for us.
Unless you spend every minute of the working day with those 'craftsmen' who are putting that van together how on earth are you going to know if they cut corners? I'm not talking about the materials used as such, I'm talking about any cock ups/bodges they may end up making whilst attempting your one off's. My point is, proper conversion companies who make X amount of set layouts will have more in the way of jigs and templates to aid in assembly, fit and finish.
I have actually had parts of our van apart, the fridge area for example when I discovered that the 240v element did not work so I know what I'm talking about here.
Here in the UK, what materials do we actually make from scratch? not a lot, most of it comes from abroad, don't mean to say that it's all crap though, same with people, often our foreign cousins work harder and have more pride in doing a good job than some so called craftsmen here.
You seem very biased about the UK, same with the whole VW T4/5 thing so no doubt I won't get through that thick skull:D I'm not saying that it's a bad van, far from it, we've had ours for 8/9 years but we have out grown it now, there ARE bigger panel vans around so that's what we have to look at.