Bomber, That is horseshit.

I took my car to VOSA for its SVA test with no tax, no test, no registration plates. Didn't even a registration number to put on a poster in the window.

When I left the test-centre the cops joined the road at the roundabout and were in front of me for a good part of the journey and they didn't bat an eyelid. Guess they are used to seeing all sorts of vehicles going in and out of there.