Seems the limit for crosswind landings was/is 15 knots. Weather forecast for scheduled landing time was often the cause of aborted missions whilst I was at Alconbury. During my 4 plus years working on the programme I don't recall too many diversions, certainly the one to Wyton stands out in my memory because I went across to assist in the preparation for it to fly out again. The weather forecast checking during pre-flight meant that plenty never took off in the first place but not many didn't come home when expected.
Just found this link which might make interesting reading, particularly for the aviators amongst you.
www.fas.org/irp/program/collect/u2v3.pdf