One way around it is too select all the over 21 players who are first team regulars and leave how ever many spaces shy you are of the 8
(for example under the squad list it says "minimum of 8 players trained in england: 5 (5 places))
Because under 21's as of a certain date (mentioned at the bottom of your squad list) are selected any way ...
Sell "dead wood" aswell ... keep only first team regulars and re-invest in english talent (Shawcross, Rodwell, Delph, Gosling etc)
Hope this helps Jake