This problem was faced in the 70's, British speedway had a season without foreign riders. It didn't IMO, alter the quality of the racing, but there was a strong 2nd division with British and commonwealth riders who filled the gaps in teams.this is no longer the case as all teams are now dependant on foreign riders.(catch 22)
Because of ECU regulations it would be difficult to enforce a limit of riders as they will complain about restricted practises. The other problem will be promoters saying one thing doing another,they all want to win.It would need someone with real control to make it work (BSPA IMO not)
The best way is to encourage promoters to run junior teams,run in the interval, so the assets they gain remain with the promotion.If you don't produce your own you buy others.When there is a larger base of riders to select from then you can reduce the oversea's riders.
IMO there are not enough British riders to remove anyone from the league setup without effectingl the quality,but promoters must be encouraged to train prospects as none will do so on their own.