Just so everyone understands, the bit about 62% is due to a sneaky rule whereby your ‘personal allowance’ is reduced for salaries above £100k. The 62% only applies to that portion between £100k - £120k (maybe a slightly different value but the effective tax rate is correct). If you earn £122k and put zero into your pension and buy no shares (neither of which is very sensible) then you will take home £6198 per month on average. If you are flying loads of summer sectors (ie 100 hours flight time) then that sum could go upwards. It is not sustainable and you will fly less, and therefore earn less, over the rest of the year. The £6198 is the average figure over the whole year. It is very inefficient tax wise but you can do it. The only way to increase that amount is to have a non-standard tax code, which is possible if you paid too much tax, for example, the previous year.
Regarding £146k a year, that is not possible for a first year captain without loyalty bonus payments. It will take you a few years to earn that.