In the UK (which my assumption since they were being paid in £) most bank transfers are effectively instant.
I obviously can only speak of my experience with banks here in the USA.
Personally in a meet up situation I would only accept cash.
For the OP just some general guidelines for selling private party from my flawed personal perspective,
All sales are final and I never, ever accept returns.
Use a 3rd party to sell with, i.e. Swappa, Ebay or another type of site that is large and verified.
Avoid Craigslist, and other meet up websites.
Never, ever use Zelle or other bank transfer systems as once the money is in another account it is essentially gone.
Use Venmo or PayPal if you can. Even though a lot of people have said they have had issues with buyers submitting complaints if you are very specific in the sales listing that you do not accept returns and the buyer is buying a used item "as is" then as long as you send it insured with tracking and take pictures of the item before you send it and boxed up then it is harder for such claims to be made.
Take lots of pictures of everything and list very detailed the condition of your device even if it may affect how much you can get.
Cash is always king if you meet in person. Let the person try out the device. Explain once the transaction is complete they will have to contact the manufacturer for aby support issues. All sales are final and No sale until you have the total amount in your hands or in a PayPal, Venmo account that you can verify with your own device. Never send anything without full payment and until the payment clears and is completely deposited into your account.
If you stick to the above guidelines you will reduce your risk as much as possible.
Again we have all been burned at some point either buying or selling second hand. It sucks. But you learn from your mistakes. My recommendations are from the various mistakes I made and then modified my behavior. Of course things are Always changing and I am always learning.
Good luck to you and let us know if there is any change or updates to your situation!