I was with O2 for about 8 year, I left around 2-3 years back as they totally pointblank refused to even attempt to negotiate or make any deals with me. I showed them the deals available online to new customers with 0 customer loyalty, and they made no offers / interest, so I moved to T-mobile (then moved to EE to get 4G
)
During my 8 year stay with them historically there were awesum, we always managed to work out a deal each renewal that got me a decent new phone on a good contract, yet let them charge me enough to maintain my business, then just seemed like it all vanished and their interest in current customers was dumped.