At easyJet, you get 12.5% for becoming a Line Training Captain. After that you get 2.5% increments for becoming a TRI, TRE and Airborne Base Trainer respectively. You do not get anything for going from a TRI to a TRI(A) (allowed to teach Base Training in the sim for ZFT guys and fly first 8 sectors of line training after the sim). Therefore the maximum you can get is 20% extra. I am told, but cannot verify, that there is a further 2.5% available for 'other duties' like being a RETRE or Training Standards Captain.
Finally, I believe the training pay is better at Virgin - these increments are increased by 2.5% for each stage (ie 15% for LTC etc).
There you have it!