No, not all classic British motorcycles have engines at an angle. However, it is true that many classic British motorcycles, especially those produced by Triumph and BSA, were known for their engines being tilted at an angle.
The design feature of angling the engine was primarily done to lower the motorcycle's center of gravity and improve handling. It also allowed for better weight distribution and provided clearance for the rider's legs. The tilted engine design was often referred to as the "forward lean" or "slant" engine.
One of the most iconic examples of a British motorcycle with an angled engine is the Triumph Bonneville, which featured a parallel twin engine with the cylinders tilted forward at a slight angle. Similarly, the BSA A65 Thunderbolt and BSA A10 Golden Flash models also had their engines inclined.
That being said, not all classic British motorcycles followed this design approach. Some models, such as the Norton Commando and the Velocette Venom, had vertically aligned engines. Each motorcycle manufacturer had its own design philosophy, and while the angled engine was a common feature among certain British brands, it was not a universal characteristic for all classic British motorcycles.