For a first offence with a reading of 50 in breath the Magistrates sentencing guidelines advise the imposition of a 12-16 month disqualification and a Band C fine. You should also be offered the opportunity to attend the drink drive rehabilitation course which will reduce the length of your disqualification by up to 25% upon completion.
Normally you would be bailed to attend court for this offence, in which case you cannot enter a guilty plea in writing and you will have to attend. Even if the Court have issued a summons they would still almost certainly insist upon your attendance at the hearing. You can present mitigation to them to try and keep any sentence down to a minimum, this can be based upon things such as your reliance upon your driving licence and what impact the disqualification will have.
If you are interested in representation you are welcome to contact my department on 0333 999 7158.