The VMCC shop, tyre section, advertise a 300 x 20 inner tube for the Ensign Universal tyre at £14. I have no idea of the quality or where these are made but 300 x 20 tubes seem unavailable anywhere else at present, unless you know differently.
some years ago i fitted a 21" rim to avoid this problem but with this solution i am also not happy, the bike is to much up at the front. will go back to 20" rim again.
Regards, van Drenth JC