A big +1 to Dave's advice there.Please PLEASE see an accountant before you do anything. Do not take my (or anyones) word as gospel online.
The general advice we've always found (ranging from accountants, to google, to revenue themselves) is that as long as the relevant VAT is being paid/accounted for, it doesn't much matter on the 'technical' side of it (e.g. using a business account marked as personal, as the same domestic rate will apply). So if you're not VAT registered, signing up with a 'personal use' account (where the VAT is automatically charged) was a fine solution.You actually CAN run your PPC campaigns with Google without a VAT number. You need to select "For personal use".