I don't think there is a huge issue with training at 40 years of age, but I certainly wouldn't go into it 'lightly'...
I don't know your personal circumstances, you could be a 'Rothschild or Hilton' family member for all I know but I would have a long hard think about what is a realistic aiming point for your flying career. You're going to invest a huge amount of money/loss of earnings/effort that is without a shadow of a doubt a huge achievement and very satisfying - but in terms of your earning potential for someone who doesn't have a PPL yet?
Yes, you could argue that by the time you're finished, the new 'upwards cycle' will maybe have begun but take some time to study the industry as a whole in the UK...44/45 years of age with a fATPL/<200 hours - the next cycle could be something we've never seen before, economics have changed with airlines prefering 20 something newly trained pilots prepared to work for nothing from 01st May to 31st October, "see you next year then, we'll let you know when we've worked out our estblishment figures"
Airlines are shrinking right now, this next cycle might just be enough to soak up experience/Middle East returnees! Not to mention at the very least, all those very competant guys/gals sat 5+ years in turbo-props wanting to move up the ladder...
Your aiming point could only be as far as a turbo-prop, they'll love your life experience and a command is a possibilty, providing recruitment is good, within a 3 year period and 1500 hours/ATPL issued...but terms and conditions are not as good as the bigger players and salary at FO level is bad £22k ish - you're only ever going to be paid what you're worth...
Now you may say "I just wanna fly..."...maybe not at 40...but i'd becareful, it's just a job when all said and done and the first few years you will live to work, then the shine will eventually come off and you'll join the rest of us in been very very privilaged in loving your job...but you'll work to live eventually...and pushing 50 earning £22k or maybe not making it at all could be harsh...
And on that cheery note...
Goodnight
JB