Am currently fairly happy with O2; I’m weirdly in a position that it’s a pound cheaper to be on pay as you go than the same plan on pay monthly!? (I’m on the £15 Big Bundle.)
Considering I also get 10% back on my top-ups, if I class the credit I’m awarded back every three months I’m actually £2.50 a month better off on pay as you go.
Signal has been very good; I was with iD before who were cheap, they were also p*ss poor signal wise with MASSIVE congestion issues. A simple call would sometimes take over thirty seconds to connect to a dial tone and I could barely even load simple webpages, so they had to go.
Overall I would definitely recommend O2; I’d also recommend checking out the pay as you go tariffs as well just in case they work out cheaper too.