The Brigadier explains to the Doctor that as a safety precaution, the UK was chosen as a neutral country by the world's superpowers (the US, Russia and China) to hold the locations of all of their nuclear weapons.
While this may seem highly doubtful for the Earth of the 1970s, it has already been established by this point in the series that the Earth of Doctor Who's universe is not quite our own, but with differences - including manned missions to Mars, a third BBC channel, and, the following year, the revelation that the England of the time has a female Prime Minister. As a result it's not impossible that such variances would produce an Earth where the political set up is subtly different to our own.
While this may seem highly doubtful for the Earth of the 1970s, it has already been established by this point in the series that the Earth of Doctor Who's universe is not quite our own, but with differences - including manned missions to Mars, a third BBC channel, and, the following year, the revelation that the England of the time has a female Prime Minister. As a result it's not impossible that such variances would produce an Earth where the political set up is subtly different to our own.
The impressive looking robot costume clearly has some unfortunate issues with its large heavy feet, causing the actor to stumble a few times, and at one point it requires some none too subtle help from another actor to manage to negotiate some stairs.