If you bought the car from a garage, you'd think they might have told you that the car was previously registered as a disabled users vehicle, especially considering the fact that it would have an effect on your ability to use the vehicle legally on the road and to actually drive it away from the point of sale.
It's probably too late now, but in future, make the problem of vehicle tax the problem of the organisation selling you the vehicle instead of your own. The market is such that many private buyers will barter tax with the seller, especially if it's a business. Next time, tell them you want it taxed if it isn't already, and if it's not it's going to cost you time and money, so you'll want that taken off the vehicle's asking price.
Seems shoddy to me that they've sold you this vehicle with no tax and failed to inform you. Since you're not disabled and are legally obliged to tax it, was it really sold to you "fit for purpose"? Because you couldn't drive it away...