I think the CAA should close the "loophole" (or perhaps it should be open a new loophole) so that airline pilots could instruct in their (masses of) spare time. I never did my instructor rating although I flew with many new PPLers and lost count of the number of people who said I should instruct. The rating is expensive though, and for us lucky enough to be working for the airlines, all instructing hours count towards maximum duty and flying limits, as they are "commercial" flying hours, whereas flying yourself about using your PPL isn't. It is a shame as I feel that it doesn't affect the day job as you do not suffer anything like the same fatigue instructing, as it is a welcome change from flying the big jets.
If the CAA could address this problem, it would help. There are many, many airline pilots who have lapsed ratings but given a push would do some p/t instructing, if the company would allow it.