I am a hold pooler and these are my thoughts -
Assuming that you aren't a captain, or expecting a command within 10 years or so (as I believe is the case at Monarch for new joiners?), then put it this way - I know of no UK airline with a higher starting salary for an FO. Then that salary rises every year.
I have seen the pay scales and they look pretty decent still, despite the new scale. Not low cost European base salaries by any stretch of the imagination, but great relative to other UK based jobs.
As always, it depends on personal circumstances with respect to command, cost of living where you are etc. But it looks to me that BA is still the best paid job in the UK when taken at face value.