As others have said ...best advice is to go to the Police station with no excuses ,,explain you did not open the letter at the time (you don't have to say why) and that you would like to apologise for the oversight and what could be seen as wasting Police time. No need to say more. If you are honest with them they may just give you a warning and points on your licence, best case scenario, and maybe a fine, Pay up (or ask for time to pay per week if you cannot afford to pay all at once) Meanwhile, your local CAB may know of a Citizens law centre where legal advice can be given free (my daughter did work placement in one when she was training to be a lawyer ...normally they are staffed by qualified supervisory lawyers) ,,,,but I dont think you should panic into employing a lawyer at this stage. If, and it is very unlikely ...given 'first offence' good character, apology, etc , it does get referred to Court ...don't panic ..but then you can ask CAB for advice ...and take a friend for support !
Good luck !
System here is different ...I had a speeding fine for doing 70 in my own road (50 limit) simply without realising ...letter comes through the post taking three points off my HUSBAND's licence ...and sending a form which you take to the local newsagent (I kid you not !) and buy a special stamp, stick the stamp (90 euros !) onto the paper and send it back, After six monthe another letter is received returning your licence to clean ! ......my husband prompty told my neighbour's daughter (who is an officer in the Gendarmerie) and I said DONT TELL HER THAT I will be placed under Guarde a vu (arrest !) but she just laughed.....luckily !
Bon courage ! 