It's always worth filling the form in ! You have more chance of getting into an orange or light blue 737 in 18 months if you've got commercial experience on a TP working for a commercial operator. 20K is crap but it's better than nothing and the experience will be advantagous when the real jet boys start recruiting. In reality there are a lot of people out there with a lot of experience on serious types who are looking for new jobs. Without any experience the chances of getting into that shiny jet are limited. A friend did 18 months on the ATP with BACX and got straight onto a nice orange 737. Any commercial job is good.
Don't expect much from BACX there is no secret backdoor to BA and that is likely to remain the case. The company changes its plans from one day to the next so don't expect any nice secure warm fuzzy feeling. Lots of unhappy people looking for new jobs so there may be limited opportunities (lots of people already waiting for those though) on jet fleets. But do you think the RJ and 145 are progressing your career. Maybe maybe not !
Even if you get no further than the interview and sim check that experience alone prepares you for the next one. I wouldn't have got my job unless I had previously failed another interview from which I learned a massive amount.
All experience is good - go for it.