Technically the test centre should give you your new MOT certificate, you should then hop on a bus or whatever to the Post Office and get a new Tax Disk. Then go back to the test centre and drive away your car.
If you want to chance it, you could drive your car away from the test centre, but there is always a chance of being stopped/caught on the way to getting the new disk.
As I've just said in your other thread, you can sort out an MOT up to 1 month before it expires and they will post-date the new certificate to still last for 12 months from the end of the current certificate. This would mean that you avoid this problem, as you could have MOT'd the car weeks ago and driven it away afterwards. Then you could have got your Tax today, or whenever and your MOT (that you sorted weeks ago), would still have run for another 12 months from today.