CRUG - unlikely to be much of a problem to the mobile phone network if going up the A1 at 100kts.... bear in mind how many people travel up and down motorways at 80/90mph with their phones continually hunting for the best signal.....
The fact that you would be getting more base stations in range at a higher altitude (and therefore overloading the network) is also not relevant, as the aerials have a noticeable downtilt (not much point in transmitting upwards for a mobile network) so an aircraft above 1000' and under 150kts would definitely not create any problems for the network....
A fast jet at 250' could cause problems if Flt Lt 'jump jet' Jones decides to phone a friend............