The only places I know of are The Model Rocket Shop who tend to sell low and medium power components, and Rockets and Things who sell mainly medium and high power components.
If you are looking just for low power, Wheelspin Models sell a range of Estes parts. Their rocketry accessories are not obvious from their page so it's best to search in order to find them. Their delivery charges are very competitive.
Rockets and Things sell 34 inch long motor mount tubes in 24 and 29mm diameters. They also sell couplers and centering rings. There is also a range of larger body tubes.
The Model Rocket Shop has a good range of smaller body tubes, mainly Estes.
Wheelspin Models also sell Estes body tubes, smaller motor mount kits, and standard motor retainers. They also sell Estes plastic retainers for 24 and 29mm reload able cases. At around £6.50 they are the cheapest I can find in the UK.
For engine blocks, I usually cut a length from an old spent motor.
Hope that helps.
If anyone knows any other outlets in the UK then I'd be interested to know as well.
SO
PS. I've also looked at the cost of US/UK delivery. Unfortunately I can't swallow the cost, it stick in my throat.