I could argue you can train from EBBL too (Muse has done it with Tansey) but that is even harder and definitely not possible to train 2 players from scratch.
By the way I have trained both Tansley
and Pattison (and both of them could reach NT one day) since their 18 yo and I've never been in danger of being relegated.
The main problem I see for England is its lack of teams which train english players for NT. Actually, there's only two: Tide of fire's and mine.
Our NT is becoming old, especially on PG/SG position whereas our SF position seems pretty weak (except for Marshall). If we want a strong NT, only MVP, HoF and ATG potentials (and some SP amazing 18 yo) must be kept in the database and followed.
Moreover, keeping 33+ yo in NT instead of promoting young and a bit weaker, but still in progress, players can't encourage managers to do sacrifice for NT.
NT is a question of balance between experienced players like Marshall or Simpson and 24-29 yo players who are still trained. Don't forget future generations ;)