Nationwide will know more about building society cheques than bank drafts, but they are similar things. Yes, some people forge them but otherwise they are difficult (but not impossible) to cancel. A number of steps to consider for this transaction are:
What is the value of the transaction? If it's a few hundred pounds, cash is usually easiest to use.
What sort of person is the buyer? If the buyer is willing to hear your thoughts on the payment process and is willing to be a little flexible, then the transaction is less likely to go wrong than if you are dealing with someone who insists strongly on one particular payment method (perhaps supporting their stand with some complicated story). If a large sum is involved, neither buyer nor seller should be wanting to rush things.
Can you complete the transaction in a bank? If you are receiving cash (or even a cheque/bankers draft from the bank that you are in), the cashiers are likely to notice any forgeries when you pay the money in as well as providing a fairly safe environment.
When you pay in a cheque/bank draft, you can ask your bank to "rush" the cheque (some banks have a different name for this) so that it clears within a day or two. There's no guarantee of the time or of the cheque not bouncing later.
Is the buyer providing good proof of ID for you to fill in the V5? You don't need to check ID (you can just ask for the name and address if you want).
There's no one answer to give but you need to look at the person and the amount involved when deciding what to do. You certainly shouldn't hand over the keys and definitely not post off the V5 until you are certain that you have the money.