RoboBrix are modular robot building components - with Wheel Bases, Grippers, Servos, MicroControllers, Sensors and an ever expanding line. In addition - they are mechanically compatible with the Lego system.
These have been in the pipeline for a while, and it is great that they are now commercially available, with more of the line on its way. I hope to convince a UK stockist to consider them.
They are available for purchase currently at their home page - http://www.robobrix.com/ via The Robot Store.
They require soldering and some assembly. At their price they do compare fairly favourably with other Robot Kits, and in terms of electronic expandability beat the Lego RCX hands down. If you already have Lego and require a controller - I recommend looking at these first.
Time to build your own robot! Using a Raspberry Pi with parts and some time, you can use my book to learn how to make and program a robot with automatic behaviours.