In the UK, installation, bases and navies are very close to or embedded in cities. PJHQ is Northwest London. The Royal Navy is based near large cities on the South Coast and Scotland. Thermonuclear attacks on these bases would affect millions of people.
The location of the UK's current nuclear submarines bases, surprisingly close to Glasgow - the largest city in Scotland, was mainly due to the preferences of the US Navy.
The UK Government had agreed to US submarines being based here and wanted them somewhere reasonably remote (Scotland) - but the US Navy also wanted to be reasonably close to airfields and other facilities. So a site at the Holy Loch was selected for US Navy use and the Royal Navy decided it should position its bases next to the US base.
Apparently the Royal Navy originally wanted to use somewhere much further north such as Loch Glendhu - which is fairly empty and nowhere near a major city.
Edit: US Navy stopped having a submarine based in Scotland in '92:
