The earliest calculating engines, such as Charles Babbage’s Difference Engine were all mechanical. It wasn’t until the invention of the thermionic valve that electrical computers were possible.
After the second world war, electrical computers began to slowly displace mechanical calculating engines in the corporate world. These were very slow by modern standards, and required a lot of maintenance due to the relatively short life of the valves. They were also very expensive and only large companies could afford a computer. Towards the end of the ’60s, they began to be replaced with designs based on germanium transistors, which allowed much cheaper (and smaller) computers to be built. While still expensive, these were affordable by a much larger number of organisations.
Early computers were very simple machines and a number of companies sprung up in the UK, including Wales, to build them.
Simon Lavington, Early British Computers, Manchester University Press, 1980. Online Copy