Ok i will list the things I can think off that could cause signal loss.
1. Military Jamming which can be done without NOTAM and will effect both roof and handheld.
2. Illegal Jamming some little baw bag has bought a jammer on holiday or has made one for the crack and has turned it on. Roof mounts should be better due to the airframe shielding them from it.
3. Terrian shielding. Not Much of a problem in the SE but wales and Highlands in the glens it is an issue. The roof mounts will be better but it will still cause problems as they will only be able to pick up the straight up contacts. Which as we know from the GPS sphere theory gives you the worst fix. You might say so what. But if you decide to go up the wrong glen your maybe looking at trying to turn in a climbing situation as the sides come in.
4. Wx. Anything which contains large amounts of water will block the signal.
5. EMC clutter as the previous poster mentioned (hadn't thought of this before but radius squared rule means that once your above 500ft I would be very doudtfull that it would effect either)
6. EMC hot spots as indicated on charts (bit obvious this one)
7. The yanks start mucking about with the SA on the civi signal.
8. Yanks go to war and decide to turn the civi off.
Numbers 7 and 8 are getting less and less likely. The troops are carrying more and more civi gps because the mil spec ones are very expensive. So if they started mucking about with that it would cause to much problems for allied troops.
9. Some internal problem with the unit.
10. Fault in the wiring to either the unit or the remote reciever.
Have seen this one once with a GA and many times in boats. For VFR club flyers at the weekend I think it will be the most likely.
The likely hood of any of these causes which are no way complete. Is completly dependent on where you fly. JMC starting soon up in the highlands i would suspect there will be lots of drops in signal due to the largest live fire exercise in europe blowing hell out of the north of scotland for a week. Around Leeds the MOD site to the north could be suspect (apart from the !!!!e wx). Bad wx is suspect everywhere. As for the highlands the hills arn't going to move so you still won't be able to get a signal in the usuall places.
MJ