It's an absolute certainty that the published ECF grade isn't a true reflection of playing strength as measured by results, since the ECF add an age related supplement to it. They then compound the issue by not using the grade as published for the next calculation process, preferring to treat juniors as new players. With neither cook present in the parallel FIDE ratings, these lag behind.David Shepherd wrote: This data shows that the juniors are significantly under-rated compared with adults (assuming the ECF rating is a correct reflection of their playing strength which it may not be).
The performance of English juniors in International events against their peers from other countries doesn't particularly suggest they are under-rated against fellow juniors by world standards.