Personally, I do not see the need to use a very expensive third party company.
The DIY method of SVA testing is fairly straightforward.
Your money will be better spent buying a copy of the SVA testers manual.
If you search enough. You will find SVA manuals on the net for free viewing/download.
Just ensure your build meets SVA requirements. Then submit the car for testing yourself.
If you follow the book build to the letter, the car should pass the SVA with no problems.
SVA test & re-test is going to be a lot cheaper than using a third party company
If you are unsure of any aspect, help will be at hand in this forum.
Ronnie