Not Jaguar, but Daimler.
Jag used a DOHC inline 6 (which was a true Hemi). Daimler cars, which used that little V8 were built by Jaguar and used similar body shells with scalloped grilles and slightly different trim. The Jag 6 was a much more powerful engine.
The Daimler Dart shown in the ad used their engine in a Standard-Triumph chassis, basically a TR-3 frame.