Road Bike £600 or under
Last seasons models usually go for a big discount so its worth trying to find one, are you prepared to wait if needed. Consider the used bike market if you know what you are looking at, or have a mate that can help out. Even if you don't have the skills, a LBS might have used bikes. Best bike for buck for someone who is not familiar with bikes is to buy used from the LBS. 

Any bike in that price bracket will be reliable. They get reliable as soon as you leave the department stores and go to a bike shop.  

For those hours in the saddle, far more important that any bits of paper describing the will tell you is the bike fit. Most important is go to a real bike shop, not a department store. I would recommend a cheaper bike and paying for a proper bike fitting session, decent shoes and pedals, and good bike shorts/bibs. No point spending all your money on the bike and having a poor fit and uncomfortable cloths.