Standby Towers or Emergency Control Units
Many years ago - in the early 1970s (before even MACC opened) - Manchester Airport decided to utilise an old "Runway Controller Caravan" as an Emergency Control Unit. Later, that was replaced with a portakabin-type building. Both were located near the landing threshold of what is now 05L. In neither case was any radar contingency provided so the plan was that approach control (procedural) and aerodrome control be provided. In reality, that plan was hopelessly unrealistic. Given that there was neither willingness nor money on the part of the airport owners, the idea of such an emergency control facility was knocked on the head - if I recall correctly - around 2001. [The matter had to be addressed as part of the Unit Safety Case]. The ECU building was retained as a communications point so that ATC could keep in touch with LACC during any evacuation period.
Since then, I have departed from NATS so the above information is historic. Perhaps some current Manchester bod might update us?
There is no obligation to maintain an emergency control facility (or standby tower). It becomes a commercial judgment. The usual choice is to accept the risk of loss of business in what is perhaps a once in a Preston Guild timescale.