Having done a self build 7/8 years ago, we are now in the process of selling up and downsizing to a smaller, SIPs house which will be even more thermally efficient than this one.
So we have also got a plot and designed the new house yo give very low running costs.
Obviously the company we are buying from ( 7 miles away) would be no good to you but I see Potton also sell Passivehouse Standard homes but I assume they do others as well, as they are part of Kingspan.
The house we will be building, with it being SIPs, doesn't comply with Passivehouse which I'm sure you will know about, but they would need to be airtight which adds to the cost. We aren't doing that, another reasons was because having a Passivehouse you have to rely on a Mechanical Heating System so we're not doing that, having UF Heating, all the usual Eco stuff we have here but on a smaller scale.
Having already Self Built this house we have a lot more experience than we did 7/8 years ago but are looking forward to the challenge. I would say anyone can do a self build, just be sure of your contractors and services (plumbers, sparkies, etc) but its an exciting time once you've made you minds up.
With regards to Potton, without knowing them, the least you should do is go and see one of their houses including one being built if you can, to see what they are like during a build, also try and recommendations from current Potton owners if you can. Just do a lot of research to make sure you are happy with what they can offer before setting off.
Although we got out plot off a local land owner, one we used to look at is
www.plotbrowser.com which if you haven't already seen has a lot of plots in Englandshire so maybe wort a look if you haven't already/
Good luck !!
www.russquinlan.co.uk