Pilotowl,
According to the CAA stats, there are 12,000 ATPLs, 3700 CPLs and , so say 14,000 airline pilots..ish? Out of those, say 3000 aren't working for one reason or another, so say 11,000 pilots in total. Almost pure guesswork, but possibly not far off. Must admit, I thought there were more, but there you go.
BA has 232 aircraft. I would guess at an average of 6 crews per aircraft which is 1392 pilots. Extending the 2.5% rate who reached age 55 this year (from the CAA stats) and assuming a static growth then that is a requirement of about 40 pilots per year.
BMI has 41 aircraft. Assume avg 5 crews per aircraft, 205 pilots, 5 retire each year...at a guess.
OAT supplied 35 to BA and 5 to BMI this year. I can't be arsed looking up the rest, but I think there is sufficient to suspect that OAT have cornered a reasonable chunk of the market.