When I brought my car from Honda around 14 months ago, and this was a main dealer Honda they were adamant I was to have a valid Tax disc "BEFORE" the car left their forecourt.
To try and speed the process up a little, as I needed an insurance cover note, I had my insurance company fax my cover note. Honda wouldn't accept a faxed copy as I was advised that any taxing station will not take a Faxed cover note or insurance policy, despite my insurance company stamping the cover note with their approved notice and calling the dealer to say I was fully insured, and legal to drive the car away the next day. I even tired myself to tax the car, and my local post office wouldn’t take the faxed cover note.
Honda had none of it, followed the book, and wouldn't allow me to take the goods I'd paid for (edit - Honda paid for actually HP- Ha) until it was fully legal. Try as I might, I was informed every time, without my valid posted copy of cover note, and valid tax I couldn't take the car as Honda were not liable to sell goods that go against the UK traffic laws.
Got to hand it to them, when I drove my car away, I was fully taxed, fuelled and insured. So, if I did have a mishap, I was fully covered, as Honda even gave me a full service and pre check certificate saying the car's lights, tires and all required items were road legal. Stamped by Honda, so as to ensure me, and to satisfy any claimant that my car was legal on the day of purchase. Even though the car was only 1 year old. Now that is service, that is how to sell a car, and they were right, you shouldn't drive a car without valid tax, and the car giant sales personnel are "Wrong" to advise you can.
As to if you can or not, or you will is another matter, but I'm with my Honda Dealer and I wouldn't do it.. I was educated when I brought my Honda, as in past I’d have tried to do what you are doing, but after dealing with Honda, I know this is now wrong. There must be something illegal about it, as if Honda were scared to allow me to drive my car off their premises, then something is a miss.. Upon saying that, I was buying the car on HP, so technically, it’s my car but Honda still own it. Perhaps this is why.. LOL