You are applying for a minimum of 9 years in the RAF, so:
1. Why would you want to join for that amount of time in a specialisation you don't enjoy?
2. What will you say in interview when they ask your reasons for joining the RAF and your choice of trade?
3. Who knows what the situation will be outside in several years time?
Sorry to sound negative, but I'm trying to understand where you are coming from with this question. You say you want to be a mechanical technician then ask other people what you should do. The RAF will give you qualifications that are recognised outside, but you would then still need to get your licences for aircraft work.