20 pounds is about 30 USD, right? If that's close, the quality of play won't be good at all in the live tournament. That's a pretty small buy-in and will attract mostly recreational players. It will more closely resemble a free roll than a live tournament you see on television.
If you've heard that online players are generally better than those that play only live, you probably heard that from online players. I think the format (live vs. online) has very little to do with the quality of the player. The one difference that may be valid is that online players can see more hands over time than a live player. But if you're only playing micro stakes online, bigger stakes at a live tournament will be a different animal and you may be at a disadvantage.
Anyway, with the tournament you're planning to play, don't worry too much. Just play and have a good time. As far as etiquette - if in doubt, ask the dealer. If you don't play live often, it's also a good idea to clearly state your intention (check, raise, all-in, fold, etc.). Don't make the dealer or other player guess what you're doing. Also - take your time when betting. Online it's very easy to figure out what 1/3, 1/2, or 3/4 of the pot is. Live, the pot size is harder to keep track of.