Without a B + E test you can only tow a maximum of 750kg regardless if whether it is loaded or empty. I am not sure if you can use L plates if you are under instruction from a person with said test certificate
you can tow over 750kg with L plates with someone sat with you who has either taken the B+E test or took their test before 1997 so automatically has that entitlement. you are limited to the weight you can tow though - think it works out as one horse, but it's difficult to find out for sure - you get different stories from different people.