THE constant threat of terrorist activity makes security as much an issue in Britain as in India. John Major and his shaken cabinet members had to duck under a table when a rocket was fired on 10, Downing Street from Whitehall. Margaret Thatcher escaped a bomb attack on her hotel in Brighton when she was prime minister. Bomb blasts are an occasional occurrence London has learnt to live with. The security cover around British government leaders and the royalty is thick, but much of it is not visible.