If you look back at those that have become astronauts in the past, they are nearly all pilots, and military pilots at that. The reasons for being a pilot are quite obvious - you need to be able to actually fly the equipment, and you need to do that whilst under the effects of gravity which military pilots will have far more experience with. Also, it is generally military pilots who are chosen to become astronauts as they have the security clearance needed for many space programmes which are nearly always carried out by government agencies. Military pilots are the majority of astronauts because they are considered the very best of the best - you wouldn't want anyone less than that in command of the multi-million usually multi-billion dollar equipment.
To become a pilot in the RAF, you can find the minimum requirements on the RAF careers website really easily. They will seem very low, but the training is very challenging and so you would have to question whether meeting those requirements is really enough.
However, as we can see with things Virgin Galactic, space is now becoming a commercial enterprise with commercial personnel. So I suppose it will become more likely for civil pilots to go to space in the future. However, pilot training in the commercial sector costs tens of thousands of pounds, whereas in the military it is free. Also, you cannot be more badass than a fighter pilot (apart from possibly an astronaut).