Didn't say much about how much electronics experience you have, whether you arel looking for a quick/cheap/simple solution, or built from individual components.
Dr. Who has been around a long time, might already be a similar product that you could improve, search E-Bay/UK.
Should be plenty of Dr. Who models in stores, have seen some in America, so the UK must be floaded with them. Good source for your robot base. Something you need to consider before getting to the mechanical portion of the robot, which will need to fit inside, and be powerful enough to move it around. If there is a large assortment of sizes and styles, you might be able to find a robot kit that fits in nicely.
The voice, plenty of choices and sources for record/playback chips. Not very expensive or complicated. Never used them, but heard there are several talking toys and keychains that can be taken apart and reused.
Just a few ideas on how I'd go about building this, use a search engine to locate what you need.