The change from 17.5% to 20% wasn't overnight. Everyone knew it was coming for a long time. It's unlikely there'd be a snap change which would catch anyone unawares.
I remember the massive 1979 increase was effective the Monday following the budget speech. The point is that it’s a sum decided on by the government over which the person preparing the quotation and subsequent invoice has no control. If a company quotes £6k all inclusive and the VAT rate goes up, they’d be relying on the goodwill of the customer to stump up any extra.