According to Wiki, a Venom was used for the initial trials of Firestreak.

One company making airframe, engine and armament is showing off...

de Havilland, prior to being absorbed into the Hawker Siddeley group at the UK Government's insistence, was typical of many UK, French and US manufacturers involved in various branches of aviation and weaponry.

The times were very different and competitive, not just between nations but between companies in each nation. Computers were a thing of science fiction, slide rules and teams of draughtsmen were the order of the day. In the UK Most airframe manufacturers built their own engines at some stage, many built dumb and, for the times, smart or at least somewhat guided, weaponry. Test pilots were exactly that, pilots testing to and beyond known limits,with the accompanying loss of life. Sometimes, as with the Comet, there were deaths in civilian service once new airliners were certified.

Aviation history is riddled with might have beens, most just failed financially. Some, like the Comet killed more than the bottom line.