In 1971 the government's civil defence budget was slashed. Local councils limited their building of conventional nuclear bunkers and focused instead on cheaper alternatives, for the general populace anyway.
From government public information literature:
"Though secure, Britain's existing nuclear bunkers are cramped and do not offer the comforts of home living. Built from drab, uninspiring concrete, they are brazenly unaesthetic and because they lack natural light, occupiers risk becoming fed up during their stay.
"Government officials, royals and the independently wealthy have courteously opted to endure these cheerless conditions so that you don't have to*.
"We think that you and your family deserve more agreeable accommodation when that 4-Minute Warning jingle sounds.
"Enter ALLCLEAR, the government's new post-nuclear solution for YOU. ALLCLEAR Open-Air Bunkers are easily erected in approx. 60 minutes. They are light, airy and available in a choice of fun colours: orange or yellowy-red. They even come with their own FREE power-string, velcro all-seasons flap and a multipurpose stick [...]
"*Please note that any attempt to secure a place in a traditional concrete bunker (and in doing so compromise the well-intended benevolence of the abovementioned persons) will be rejected with extreme prejudice."