The problem is that BSP only applies to a specific types of pipes, soft water piping like PEX, gas and other pipes are often a mishmash of thread standards as they are not covered by BSP.

So in reality there already is a mixed system which is often what causes a lot of headaches, what’s worse is that BSP is also a mixed standard it has both metric and imperial measurements for its standard threads however when you mix fittings that were machines using metric and pipes that were machined with imperial I’ve found that because of slight tolerance differences you often can get into a point where stacked tolerances becomes and issue.