In the spirit of encouraging players to return to the Megafinals year after year, how about a UK Chess Challenge grade. It could be something as simple as:
UKCC Grade = 6 + 2 * (sum of Megafinal scores from previous 2 years)
so grades would range from 6 to 30.
The 'extra edge' might incentivise children. The other benefit would be to help achieve sensible pairing, especially in the first round - too few children have a regular grade.
There was an instance last year where a club player with a grade who had scored 6/6 the previous year was paired in the first round against a casual player without a grade who had scored 5.5/6 the previous year. A UKCC grading system would have avoided this.