I know in Xfactor they have to do 3 auditions including all the dud acts .
I wonder is it the same for BGT.

Deliberately picking duds to go through and then be mocked, at the expense of I'm sure, a lot more talented people, should not be allowed.

Those dud acts just don't walk in off the street as Cowell would like us believe. He must ask his pre-audition team to pick a certain number of dud acts for each show.

Quiet a lot are contacted and invited to audition in the main show.

Depends, nowadays lot of the auditions are online, you upload or link to a video and then they'll invite you, rather than you going to them.