No true Londoner can resist this. When walking over Tower Bridge you just have to stop, put one foot on the north side, the other foot on the south and then wait for a bus to pass. Being a bascule bridge (French for "see-saw") you are left with the feeling that this bridge was never built for such traffic and you are about to fall into the Thames.
Contact the Bridge Master at the following site should you want the bridge raised.
I assume you have to be in a boat and not just standing astride the gap in an elberate bout of brinkmanship