There's an ongoing problem with the VAT system. When one VAT-registered business is selling to another VAT-registered business, the transaction is generally "VAT-neutral". One company charges VAT to the other company, which pays it to them. The first company gives the VAT to HMRC, the second company claims back exactly the same amount from HMRC. Overall everyone is in the same position. That's how it's supposed to work - at root, VAT is a tax on consumers, not on businesses.
There's a problem there, though. What happens if the first company doesn't give the VAT to HMRC, but instead shuts down or goes missing? The second company will still most likely claim the VAT back from HMRC, and get it. In that case, the public finances (we) have lost out. That wasn't a VAT-neutral exchange.
This "missing trader" fraud used to be a particularly big problem with the import and export of computer chips and mobile phones, where it was relatively easy to create large purchases and sales of those commodities, and make off with one half of the VAT. That was known as "carousel" fraud. You don't hear about it so much today - presumably HMRC are largely on top of it now.
Another industry where HMRC clearly think it's a problem is the construction industry, where they consider businesses are similarly prone to disappearing, along with the VAT they've been given by a customer. So, next year, in October 2019, the way VAT works in construction will change. In the kind of example above, the supplier will no longer charge VAT to the customer. Instead, the customer will just put both the VAT on the sale AND the VAT on the purchase on their own return, just as an academic exercise, cancelling each other out (this sounds redundant, but is necessary since there are occasions where someone is blocked from getting all the VAT back on their costs). So the supplier no longer has any VAT to run away with.
This is an excellent idea in general, though it's fair to say that not every trader in the building industry has totally bomb-proof accounting systems, so in practice there's bound to be a bit of confusion to begin with!