I know this is an old thread, but I thought I would comment in case it helps anyone reading it at a later date! So many great comments and I completely agree. As @Fallingditch said.... we have found the structural engineer the most difficult. With delays of a year and a half now and all the problems are still not ironed out.
Our architect was amazing and actually quoted us a fixed price on a no win no fee basis, although I do agree with the build cost element. We gained our house under the "paragraph 55" clause in the NPPF and it was meant to be a 200k build, but it's more like a 300k and that's with hubby (who is a builder) doing pretty much everything and we are project managing etc....
Our problem is that the same architect wanted 11k for building regs with structural engineer fees on top and we decided to take this on ourselves. Although we have saved a lot of money, the hassle and stress from having a crap SE and a not much better "new Architect" does leave me thinking we should have just coughed up the money as we would probably be living in our house now. We have made changes to "downscale" the build to make it more cost effective, but my advice would be, the builders, engineers, trades, that get back to you quickly and effectively (in my experience) seem to be the ones that are on the ball and efficient at their jobs. So look into them better than we did..... lol! Some corners in my experience are not worth cutting (like the SE) I wish we had chosen the more expensive one, as I know my limits and this stuff just goes over our head.
Good luck, it sounds very exciting!