I think you are effectively looking to match a shorthaul ET fare with a longhaul CW fare which is then priced ex-LON, so it's not going to produce any saving.

If you want to be certain your second search is failing because you are only looking at shorthaul legs in full-Y, and that's almost guaranteed to exceed the I-I-I-I fare. You'd have to list all the economy buckets - I think the correct coding is "/ f bc=y|bc=b|bc=h...." where you list all of YBHKMLVNQOSG.