There are several Official Secrets Acts that are in force in the U.K. The one relevant for your question is the Official Secrets Act 1911. See Section 3:
For the purposes of this Act, the expression “prohibited place” means ... any work of defence, arsenal, naval or air force establishment or station, factory, dockyard, mine, minefield, camp, ship, or aircraft belonging to or occupied by or on behalf of His Majesty ... [and there are other things that can be prohibited places, including places declared by order]
To be in violation of the Act, it is not enough to merely be in the area. See Section 1:
If any person for any purpose prejudicial to the safety or interests of the State... approaches, inspects, passes over or is in the neighbourhood of, or enters any prohibited place within the meaning of this Act... he shall be guilty of felony
There is a bill before Parliament that would redefine prohibited place (but still capturing the base you have photographed) and create additional offences in relation to prohibited places. See this factsheet and the current text of the bill. Importantly, Section 5 would create a new offence of "unauthorized entry etc to a prohibited place," which would make it an offence to access a prohibited place without authorization while knowing (or while one reasonably should have known) that the access was unauthorized. No purpose prejudicial to the U.K. would be required. However, given the lesser severity of the wrong, the maximum punishment would be six months in prison or a fine.