IM IN THE STATES AND DONT KNOW IT ALL BUT MY FRIEND FROM THE uk SAYS YOU HAVE TO MAKE A PROPOSAL TO THEM SHOWING CHASSIS DESIGN AND GO FROM THERE, IT IS A VERY DIFFICULT PROCESS BUT WORTH IT IF YOU SUCCEED. LMK IF YOU NEED MORE INFO ILL CONNECT YOU TO HIM
If you are refering to a 1 off that you've built for yourself then I believe IVA is what you have to pass.
Frankly I think you'd do better to ask this question on LocostBuilders as there is a lot of experience on that forum for getting scratch builts on the road in the UK.