I've just had a thought... Buy a load of plain white tshirts in bulk, just get a pen, and write "stop the war" on the front. Then, stand in Parliament Square and hand them out (to people who promise to put them on right away). You can't charge, otherwise you'll need a Street Traders License, but if you're giving them away, I think you're safe.
Having a load of tourists wandering about in "the zone" with protest tshirts on, doing nothing but getting pictures of Parliament and such like would make a complete mockery of the new laws.
This plan would cost you a few quid, and an afternoon of your time, possibly fending off the odd visit from Officer Dibble, but would it work?