I applied for WARBY for last year's GVBR. I had done that sort of role for the previous ATB and MSC. In the end they put Earl (who has been a WARBY a number of times) and I in the Riding Marshal position (a mix between WARBY and Route Marshal). They put me there as a "stepping stone" type role before I get the chance to be a WARBY. As for Earl being a Marshal, I'm guessing that was because there were a lot of applications for WARBY.
Selection criteria, as far a I know, is mainly previous experience (ie being a WARBY on GVBR), although they bring in the occasional new person each year. I'd suggest, if you want to be a WARBY, that you apply to be a Riding Marshal for ATB (same role), then apply for GVBR (as WARBY) and hope for the best
(it's what I did...but it didn't work too well...)
And yes - there are usually more applications than vacancies.