The Concept will either be originated with the BBC or an indipendant production company who will have floated it to the BBC.
One of the first piorities for the production company will be to organise the minimum number of machines to film the shows. This will have been done by contacting people who can get the job done. In the case of the Mechanoids series £1,000 was contributed towards build costs with accomodation and travel expenses paid for. A lot of the robots in the the first series of Robot Wars were built by special effect specialists who probably wernt working for free.
Once enougth machines are confirmed and the show is given the green light then the BBC will have put it up on their website with invitations to the public to take part.
Its great that now that Jane Root (she cancelled Techno Games and Robot Wars) has left the the BBC they can do engineering based shows again.
Its a shame that its such a Scrap Heap knockoff and that the time to build for general public entry is so limited.
I wont be putting anything together. I have a family, a full time job, a robot to rebuild and some other thing Im involved with. Given more notice then I could have shuffled some time around.