via Steve Hill on FB Anglia Military Aviation Group
".. Anybody remember the U-2C deployment to RAF Wethersfield from May until June 1975? It consisted of five U-2Cs from the 349thSRS/100thSRW(SAC) Davis-Monthan AFB, Az flying high altitude sampling missions, hence the grey camo c/s. One jet (56-6700) was lost near Bonn, West Germany on May 29th and was later replaced by 56-6716. The other four aircraft involved were 56-6680, 6701, 6707 and 6714.
As can be seen here they operated very close to the base fence, making photography very easy. This amused the Yanks hence the sign " Spys parking only " they placed on the fence ...! "