The front end of the GPS receiver can get saturated by other RF signals, and I have had similar drop-outs very close to that location when using a portable with attached ariel, albeit a few years ago. I know of other similar cases also in the Daventry/Rugby areas. BBC world service also beams out from this area on the HF bands.
So far, I've never had a drop-out using an installed GPS, which significantly shields out ground based signals.