OP, as ihpj mentioned, go for legal advice.
Your solicitor should contact relevant police department and should be able to find, why are they intending to prosecute you?
A good solicitor should be able to get some useful information from the police and should try to mention your side of story that you were not there for racing or dangerous driving.
I know somebody who had a similar letter and with legal help, he ended up going for a driving awareness course only.
Hope that helps.