When you are bootstrapping, save all your cash. Once you have a lot of customers and income you can invest in administrative infrastructure. I assume since you are 18 you do not have large assets to protect.
Ok, so to check, you don't think I should bother becoming a limited business at this point in time? (It's only £15 but I guess that more paperwork would be involved.)
well, I think the purpose of you wanting to get an LC isn't to protect your own personal assets, which is the purpose of LCs. I think you want to look more professional, which a fancy business card can do, and maybe a small office, or rent part of an office, maybe split an office with someone.
Another thing you may want to consider for LC's are tax rates. I'm not certain about how it works in the UK, but you may also be subject to higher (or lower?) taxes on your income.