18 months is already an extension beyond the standard 12 months for hold poolers. Perhaps there is also the issue of currency. An applicant out of work and unfortunately no longer in current flying practice may not have touched the controls in nearly two years by the time they reach ba. The applicant increasingly becomes a training risk as the clock ticks (they will pass, but how many sim sessions that are not available in the system will it take?). I'm sure there are many other factors.
The 300 odd pilots who got in without doing the selection process...... Well let's just say there is a very demanding 18 day conversion course with multiple exams and hours and hours of sim work then the LOE, LPC/OPC to do before you are really in.
The vast majority of DEP applicants coming from any of the quality established UK jet operators would probably pass the OCC, as have the BMI guys/gals. However by my calculation over 90% of DEP applicants fail the selection process at some stage. They can train most pilots to operate an airbus/boeing 'the BA way' but have they got what it takes to be a BA captain one day? Only time can tell.