i recently left 3, after 18 months contract,
couldn’t wait to leave as the coverage in my area was terrible, just outside bath in the countryside,
my own fault really as i should have check the signal checker,
but i found it generally not great, even on a 4th top floor flat in central london, also central bristol was sporadic
i’m back with tmobile, equally as cheap,
but they have merged with orange so you get to use their signal,
they originally said you can only use their signal for calls, but i’ve been connected to tmobile-orange signal able to access 3G.
it probably not best to ask on here which network has the best signal, as we’ll all have differing stories
best is to ask friends that live close,
or get some free pay as you sim cards and see what the signal is like,