Can't speak for the UK, but in the US if you fail the class I, it may be a career buster. You may not be able to get the medical at all after that, and if you do, it can affect obtaining insurance, employment, and your future. The reasons why you might fail play a big part in that, however. If you can't pass, but can be guided through the process by obtaining the necessary doctor visits and documentation to eventually pass, then no problem.
Do you have a medical condition that makes you think you won't pass, or are you just guessing?