Phone the court and tell them you want to change your plea. Phone TV licensing and tell them the same.
Turn up on the morning of the next hearing and tell them you are changing your plea to guilty.
The only problem will be that you lose your 30% discount on sentence, if you plead guilty on the morning of the hearing you only get a 10% discount.
The TV licensing people will ask for increased costs of preparing a trial so you must tell them that you phoned in advance to let them know you would be changing your plea - otherwise you will be lumped with greater costs.
Buy a TV licence in advance of the hearing. Look sorry. Explain that you had told DD not to watch TV and only to watch DVDs (if that is the case), and that it was a one-off. Show them the TV licence that you have now bought.
The sentence is a Band A to a Band C fine (50% of your "relevant weekly income" to 150% of it.) Which one depends on how long you were watching tv for without a licence - if you are on your third then it'll be at the higher end. Relevant weekly income is deemed to be £400 unless you fill in a means form at court, the usher will give you a form. If you're on benefits tick the box, RWI is then deemed to be £110.
You shouldn't go to prison for not having a licence but you can for non-payment of the fine, so don't ignore the fine.
And don't let the buggers in your house without a warrant! (They are currently hassling me - they know I don't have a telly, they've known this for years, and they still send men round asking to come in.)