Joe,
There's compulsory excess and voluntary excess for car insurance. Compulsory excess is the amount that you need to pay towards a claim and that's mandatory. So if your compulsory excess if £200, then you would need to pay the first £200 of any claim. For a £300 claim, you would have to pay £200 and your insurance company will pay the rest (£100). Voluntary excess is the amount you're willing to pay more in the event of a claim and the more voluntary excess you have, the cheaper your premiums will be.
Insurance excess for an 18 year old vary from company to company. Some companies might want the compulsory excess to be £250. When I was your age, my compulsory excess was £200 and my voluntary excess £300. So I had to pay the first £500 of any claim if I had an accident. I opted for a higher excess to reduce my premiums as a young driver.