I can't comment on any of the builders because I've never used them, BUT if they are a registered, legit business, they should show a Vat component on their quotation or on an invoice when you pay them.
We all know that many folk work 'cash in hand' and for small jobs, on an irregular basis, that's no big thing, but on larger projects where the builder is
likely to be claiming the materials cost against the total income from the job, he would be crazy not to include a VAT component..one audit is all it takes!