Frozen chicken 1kg from asda/tesco/farmfoods is £4 per bag, and the comments about massively water injected are tosh, 13% or less water is hardly massive.
Online sites such as westin gourmet/musclefoods tend to be £5-£6 pre kg depending on deals and are fresh.
If you can find a local butcher you will probably be able to get similar prices. I currently get my chicken from a local butcher at £30 for 5kg's, and if i buy 10kg at a time he normally does a slightly better deal. workouts out at £5.50-£6 per kg.
Quatlity wise the supermarket frozen stuff is the bottom of the pile, but not by a massive margin. If cost is your main concern then this would be the best choice.
The online suppliers and butchers quality is the same, however you dont have to wait for delivery from your butcher, and normally they will ask how you want it, ie 1kg bags, 2kg bags etc and that can save some hassle over getting the 5kg bulk packs online and having to split them out.