There is a chance your vintage Brit bikes might have Pozidrive fasteners, but for the most part they are typically standard flathead and Phillips. One thing to consider on British machines of that era are Whitworth fasteners though. Hex heads that your metric or SAE wrenches simply wont fit.
The square drive is Robertson, and I have always been amazed it never caught on inn automotive production. Torx is great, but I've watch Torx splines strip out but I've never seen a Robertson strip out.