I can only remember seeing two. I was at school at Oxford in the '50s, and saw the Wolseley 1500's forebear when it was still being tested as a replacement for the Series II Morris Minor, possibly with the Minor 1000's 948cc engine; you can only be grateful nothing came of that! It would have spoiled what turned out to be two very good cars. The other occasion was in the '80s when I saw a production prototype Jaguar XJ40 on the A49 near Ross-on-Wye, several months before it came out. I may have drooled a bit