The U-2s are transiting on an operational basis either going out for ops in the hot spots further east or on their way back to their base at Beale. This happens regularly using Fairford as a stop over.
The NASA WB57s come over for research purposes in various places around Europe and use Mildenhall as a stop over. On average this happens about once a year. I saw N928NA at Mildenhall back in 2007 on a similar mission.
If there is any link then then we would have to wait for a certain Mr Snowden to release/expose the details ;-)
HTH
Graham