Step 1 is a Class One medical before anything else. Look up the UK CAA website to book an initial Class One and do it now.
Be fully aware, that to become an airline pilot takes a huge amount of work, (and money). Things you might never have thought about such as atmospheric pressure, magnetic variation, aircraft pressurisation and world time calculations, for example.
The training is relentless, both the exams and the actual flying. Also frustrating in UK weather, (but gives you a good appreciation of weather).
I did my ATPL about 25 years ago, so am out of touch with current practice and what is now availble. But probably if you are less than 30, you have time to go modular. If you are approaching 40, you need to get your ATPL as soon as possible, so a full-time integrated school might be a better option.
Even if not full-time; some sort of school for the exams will help with explaining concepts and how to work through and solve complex navigation questions for example.
Be under no illusions, this is going to be difficult and take up almost all of your time. I went to a UK full-time integrated school in the late '90s, and it took 18 months from zero to frozen ATPL licence. And that was full time attendance, as I say, with 3 hours homework every night on top of that - no time for working as well. And your family won't see you very much.
And don't pay the whole fee upfront, you should be allowed to pay by instalments as you go.
Good luck
