Well I would say, that the "rockets"-idea might/could be a bit of a stretch, though lesser, if such possibilities were looked into at earnest by the german military (of whatever branch) prior to the war.
However, a force of short of 100 E-boats even "only" armed with torpedos, maybe together with about 2 dozen subs, Typ II ones sufficient in this case, could pose quite a ... problematic encounter for the RN on its way down south with its heavy units to guard against an invasion
IMO what's more important than actual weapon systems of OTL is the "asymetric thinking" approach,
@hammerdin brings up here, as part of a ATL development, that leads to landing/invasion attempt on the british main isle.