Model rocket motors such as Estes and Quest solid rocket motors can be bought over the counter from most model shops stocking model rockets. They are available in a total impulse of up to D-class.

Mid and High Power rocket motors are available as solid rocket motors (generally composite propellant motors) but require you to have the appropriate licences. Also available, are hybrid rocket motors. Again, these are available as High Power rocket motors. The Mid and High Power rocket motors can be bought from UK rocketry vendors.
A list of recommended vendors is maintained on the UKRA website.

suppliers, shops