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.
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. It is highly doubtful, even in the 1970s, that Russia and China would see the UK as neutral due to its "special relationship" with the US.