Though there are no indication in the UK's Offensive Weapons Act 1996 regarding ASP Baton being an "offensive weapon", I recommend that you don't take that chance.
Maybe you can check by asking a police friend casually regarding ASP, I know that the Metro Police have just issued the ASP as a standard issued item. Eg. " Hey, your baton is cool, where can I buy it?", you will get a good answer after that.
Also just to let you know, in Australia, a country having similar law as UK, ASP are allowed.
I once serve in the military, I don't need a ASP.