This happened because you didn't pay the fine. Therefore it went to court. If you'd paid the fine the matter would have been dealt with and nothing recorded on your file.

However, if you don't nominate another driver then it is automatically assumed to be you. If you don't pay it - they'll process it through the court which gets very expensive.

I think your big mistake here was not to clarify what this letter meant (the letter also explained your options) and to send back a blank version (which kind of acknowledges that you got it, so you can't use the old "I never got the letter" excuse.

So your options are pretty limited now. You've lost your chance to contest it or nominate someone else. I'd pay the additional amount, or it WILL keep getting bigger, believe me.