Sinclair's focus and interest was not computers and he honestly did not have depth in it. His focus and interest was miniaturisation of electronic equipment. He started with transistor radios and went on to calculators, mini TV, digital watches and computers. He aimed to miniaturise PCB, transistors etc and sought manufacturers and finally the boxes and the assembly. So it was no surprise that he wanted a smaller version of a car.
Though 3 American companies, Commodore, Tandy and Apple had already launched personal computers. Commodore became the template for Sinclair to work on. Sinclair's ZX80 was 3 times cheaper and was immediately successful. And he approached it as teaching and educational tool. This allowed more people to learn about computers and programming, not just in the UK but across the World. His product made the PC and programming no longer a mystery. It also led to surge in students moving to take on computing as a subject. Universities rushed to provide computing as a discipline within year of the launch of ZX80. Talk about power of influence. The ZX80 changed the World, not the Commodore, the Tandy or the Apple.
ZX81 its successor became most heavily cloned PC behind the iron curtain. And part of it was political in nature. They trusted the British rather than the Americans.