Arguably they were a little late to the party, but it's not just a simple case of 'angle = good, no angle = bad'.

The problem with the slopes is that they're inefficient with space. A sloped armor layout results in a bigger vehicle for the same interior capacity (for crews, ammunition, radios etc), and far fewer corners in which one can't put anything at all. Note that, for example, Sherman was concluded to be too sloped, so later production vehicles had thicker, more vertical (relatively) armor.

Plus they figured that any round impacting would be unlikely to impact at a direct perpendicular anyway, so there would always be some sort of a sloping effect.

That said, it's not as if no British vehicles had slopes or angles. Matilda II, for example, of 1939 had sloped both vertical and horizontal on the front hull. Even the incredibly blocky Churchill has a sloped front hull. The driver's plate is often more vertical, such as on Cromwell or Churchill, but they did tend to make them very thick to compensate.

A panel of experts goes over the different countries' philosophies towards sloped armor. 5:10 into this link. https://youtu.be/aq5wWYBvsD0?t=313