The only legal way to drive an untaxed vehicle on a public road is to use trade plates. (Then, for the purposes of the law, the vehicle becomes one which has the registration number shown on the plates, for which road tax has been paid).
If you've got a friendly local garage they might let you borrow a set of trade plates, which would be good enough to convince the average Pc Plod that the vehicle was covered by valid road tax.
However, since trade plates are only valid when used in connection with the business to which they're registered, a detailed investigation by the police would show that they weren't valid for your use.
i.e. if you simply drove past a police ANPR camera, the trade plates should be recognised and you'd have no problems. However if you were involved in an accident then the more detailed analysis that the police might carry out would show that you shouldn't have been using the plates.