Just wanted to chime in and say I have recently acquired a 7ft4 Miller Powerglide (I live in Australia; indeed surf about 15min from where Grant lives). Coming off a 9ft1 mal - basically the only board I have surfed for the last 5 years - I was very focused on the differences. I can say that I could notice hardly any difference in paddling, apart from maybe having to start a few feet closer in to catch the wave. However, there is a difference in 'glide' particularly in small waves (like 1fters). On the LB, you catch the wave and glide along, with the shorter board you do fall off the back of the wave a bit. Not the board's fault; its the trade off for a shorter board of course.
Grant's 'trick', if I can call it that, is that the thickness of the board extends right to the rails and almost to the nose and tail. If you look at most boards, even LBs, they start thinning out a long way from the edges (particularly nose/tail). Even the 7S Cog linked earlier thins out a lot. Grant keeps the width almost the way through and this increases the volume (and stablity). Plus he keeps the rocker low. Hence, from a paddling point of view, although its 7ft4 it probably actually has almost as much volume and as much surface area on the water as an 8ft4 or more.
I've never really surfed shortboards so cannot compare. Its definitely easier to turn than a 9ft board. For me its a great board - no downside in paddling and turns better (and fits in the car!). So apart from the 1ft days (I was only out there because I had just picked up the board and, as you know, you just have to surf no matter what), its an allround winner.
His boards are reasonably complex design. Thus while you may find boards with similar dimensions (eg the waterskate is not too dissimilar in shape to a Firewire Addvance, other than the tail of course), there is a lot going on that may not appear in other boards. Its not just keeping an increased thickness and low rocker, he overcomes them with other designs. He also does (gets done) an intelligent glassing job. For example, its a 2x6oz for my board, but he then puts another layer just over the leash pin and over the fin boxes, to add extra strength to those high risk areas.
I would upload a pic but the boards 'attachment quota has been reached', whatever that means.
Nonetheless, being in the UK it might be hard to justify esp given the AUD exchange rate. I guess you can just wish you lived in Australia (no doubt a bit more than you usually wish...). But in terms of looking for boards, if you were going for the mid sized boards (although I see you are looking at an LB), then keep an eye on the thickness and the rocker as well as the outline (eg: narrow tail). The Walden magic and many of the firewire boards use some of these.