Simon, I'm afraid a budget of £20 - £30 isn't going to get you anything adequate enough for your size tank, like I said earlier, the bigger the tank, the bigger the costs for everything! The best you can do is stretch your budget and work out what is better for you in the long run, either external filter, or internal filter + powerhead.
Remember, you only get what you pay for! A £30 filter will do the job of a £30 filter and is not enough for your requirements.
To be honest, by the time you have paid for a decent internal filter and a powerhead, you may find an external filter would be the better option, providing you get a good one such as eheim or tetratec