I too have experienced loss of GPS signal on several occasions in the DTY area, including in aircraft with "proper" installations. I assume that there is something thereabouts that is jamming the signal (easily done, the GPS signal strength is extremely weak, I was told it is the equivalent to the energy that you get from a candle that is positioned a hundred miles away!).
Unlikely to be the Rugby masts though, as has been said they transmiitted on very low frequencies and, in any case, the last ones were demolished a week or so back!