Whats the difference between a hub that is £2 and one that is £20?

Well, they will both provide network switching. The £2 will operate at 10/100 but the £20 might at 100/1000 if you're lucky.

Even if the hub is rated to 100MBps, in all likelyhood, the cheap one will use cheap and slow components. As such, you may in actuality only get say 60MBps. The £20 may get a lot closer to the rated figure.