Just something else to consider:
How much will it cost you to revalidate the rating? Assuming you need to do the seminar and test (as you won't have done 100 hrs instructing in 3 years of which 30 in the past year) say about £600 (£200 per year for the vailidity of the rating). This is not a great financial burden to maintain something which you might at a later date want to revisit.
If you let it lapse for some time, then the cost of renewing will eventually be higher as you will be out of practice.