hey Good for you Alex,
Definitely go for it!
Plan small and think big.
Go for a few short weekend tours like the others have mentioned... plan them carefully, act responsibly (at least on the surface) and if you didn't get into too much trouble, next time you'll be able to go for a week (half term?) then by the time you are at uni you can use the 3 month holidays for tours around europe, already having a good idea of what's involved...Then the world?
I heard a story of how a millionaire started his first business. As soon as he was old enough, 16 or so, he took out a small loan from the bank of 300 pounds. He didn't buy anything with it. just paid it back in instalments plus the interest. Then he took out a loan for 1000 pounds and did the same. then for 7000. He just put the money in his account and paid the instalments without fail.
After he graduated he asked the bank for 20,000 to start a burger van business. Because he had not missed a payment of the previous loans he had an excellent credit score (better than if he had never taken a loan) and they were happy to lend him the money.
I'm telling you this story to illustrate that trust is something you can build up over time but if you just ask for too much all at once... (you know what the answer will be). The same goes for experience at travel, cycle maintenance, etc... You sound like you've got your head switched on and if you have a bit of determination there is nothing that you can't achieve.
PS. Here's an idea. Start a savings account now and label it (Round the World Bike Ride) put in a little bit each month. While your mates are pouring their money into computer games/beers/cars/cigarettes/etc you'll be heading off for.