£400 will definitely buy you a better setup if going 2.0/2.1.
Stereo amp would probably the best idea for 2.0/2.1, however an AV receiver would be worth considering if at any point you wanted to move to surround sound. Just an example, you could get the AVR, 2 speakers and a sub. At a later date, if you wanted, you could add rears and a centre for 5.1. If you definitely do not have any intention to move to 5.1 at any point, a stereo amp would be best.