Basically the trials for the EBL and BBl teams will contain the best players from the central venue leagues around them. As well as some of the players that already play National league for smaller clubs.

Ive run trials for Manchester before and we work on everything. you have to in order to pick the best rounded players. You need to give off an impression of good attitude so when the coach says come in you stop shooting and run in straight away. This will always set you apart from those that don't. also if things start going wrong you need to keep yourself together and don't throw a fit.
Despite not knowing the players you will be with that long you need to be vocal. If you are constantly talking on court the coaches will recognize that as a quality, providing you are doing it right.

Most importantly make sure you can dribble and lay up with both hands perfectly. You won't get in if you can't and that is a guarantee.