If you have to be anything, it’s best to be short sighted when you are young. It usually improves with age.
The first thing to do is have an ordinary eye test so that the error is quantified. Note in particular the astigmatism reading.
If you do a search on this section for the actual requirements, you will be able to make a judgment based on the fact that it is unlikely to worsen if there is nothing else wrong. Obviously check for diabetes etc. before shelling out the £400
The amount of allowable correction is quite high.
I'm usually rather skeptical about improving vision with exercise, but in your case, spending time teasing the focus out more and more will no doubt help. Don’t overdo it!!