Pretty sure 5.1 will be the best you'll get for under £300, you can get some Yamaha 5.1 sets for about £80 (then buy an amp) but I'm not even sure if they will be much better than the Z-5500 satellites, they don't come with a sub either.
You can get 5.1 through HDMI.
You can get 5.1 through HDMI.