Virgin use the EE/Orange/T-mobile network which has good coverage.
Freedom Pop uses Three.
Giffgaff is cheap (I pay either £5 or £7.50 a month for more than I need, I pay higher amount if I'm expecting more usage that month) but uses the O2 network which hasn't got brilliant coverage.
In a time of universal deceit - telling the truth is a revolutionary act. George Orwell
When the debate is lost, slander becomes the tool of the loser. Socrates