You could check out Pololu. They have a few options available depending on your budget. They are good people to deal with and know what they are doing if you have questions about their products.
There is also Kits USA. They have some programmable type kits as well, and are a little less expensive in some aspects.
I have just bought my first Arduino, it seems very capable and there is a huge software library available including sensors and RC servo drivers.
A £25 board will give you 50 or so digital IO lines, including PWM capable ones. 12 analogue inputs and 5 serial interfaces.
The IDE (Integrated Development Environment) is quick and simple and seems to compile efficient native code.
you can also program the chip natively if that is what you want to do.
I don't know enough to give you a comparison to all the other uP's out there but if you want a quick win that can be as low level and complex as you want it to be later I think you may find the Arduino open source project a good fit for your needs.
I hope this helps with your research into options.
Thank you very much for the suggestions everyone,
Through my research i have seen many Arduino robot kit, in particular on this website http://www.robotshop.com/project-ideas.html