What's a good "small" tank I can have a betta in?

Hexagon tanks look nice, but they're not so great for the fish as they tend to be taller than they are wide. My first tank was a hexagon and I got rid of it very quickly. Rectangular or even round ones are much better. I'm in the UK and have never seen a heater like that one before, but it looks okay. The filter I'm not familiar with either, but check it has an adjustable flow as if it doesn't it won't be suitable for a Betta who like as little current as possible. Also, check out small sponge filters (run by an air pump and airline), which would be good for a Betta.