this happened to me a few years back when operating as a sole trader,plumbing.
Went to do a job at a large company,gave the price,no vat as my turnover was not in that range and i can still see the look on the girls face,she couldn't work out i was not vat registered,it was all most like i was a cowboy operator,she just could not accept i was not charging vat,i explained about the threshold but it just did not sink in
i did do the job.
i think it has something to do with how they can claim money back of hmrc