PS: Assuming you complete the deal tomorrow, ensure that you've got the phone number of your existing insurer (if any) with you. Complete the deal and then immediately phone the insurer to get cover for your new vehicle. Ask them to email a copy of the certificate (or cover note) to you straight away. (Downloaded insurance documents are valid when taxing a vehicle).
Go home [in your old car or by public transport, NOT in the new one!] or to anywhere with internet access and a printer (such as a public library, internet café or even the car dealership that you've just made your purchase from) to print out the insurance document. Then go a Post Office (which issues tax discs) with the new keeper supplement, MOT certificate (if relevant), insurance document and your debit card (or even good old-fashioned cash!).