Assuming you were not stopped at the time, first of all you have to respond to the "request for driver's details" by naming yourself as the driver. For that speed, unless the offence was in Scotland (where they are not offered) you should be offered an "Retraining" Course for that speed. This will cost you about £90 and about three hours of your time. If you don't fancy that, you will be offered a fixed penalty of £100 and 3 points. If you fail to accept either of those, the matter will be heard in court.
Check your insurance documents. If you have points imposed you will need to tell them. Some insurers need to know immediately, others at renewal.