Flying is Fun has given some accurate time estimates on completing your PPL. Whilst I was in full time instructing the quickest PPL I did in the UK from start to finish was 3 months. This was with 2 students, who were full time and went through in minimum hours. This was mainly achieved due to both guys who were committed and had a good degree of continuity to their training. However, if you are holding down a full time job, then as FIF said 1 year to 18 months is more realistic.
To share my experience, it took me around 12 years from no flying experience to airline pilot. I obtained my PPL in 1995 and had my commerical licence in 2004. The subsequent time was spent instructing and trying my hardest to get an airline job. The bits in the middle were spent at college, working full time, saving dosh and doing what I had to do to keep current.
Keeping your licence or should I say your single engine class rating current is pretty straight forward. You may want to consult LASORS for all the CAA info, but in a nutshell, you can revalidate either by test or experience every 2 years. There is certain criteria which you have to fulfill. Your licence is valid for 5 years. To renew this is simply a paper work exercise with the CAA followed by payment of course. Once you have gained your IR and Multi engine piston class rating, you must renew these every year, and you can only do it by test.
One thing you will find about this industry, is everyone has there own story to tell, about how they got where they are. Alot of my colleagues (new starters) within the airline I work for, are between 19 and 25 years old and have come straight from Oxford after to completing an integrated course. Quite a number of people, myself included were not able to afford this, therefore we went down the modular route, which took a little longer, however, from my own perspective, I learnt a tremendous amount along the way, and have developed numerous skills from the experience.
Anyway, I digress. If I was to give you some advice, and again this is not the right or wrong way to do things, I would try and go through the modular route with a fair amount of consistency. If you can, save up a load of dosh, say over the next 2 or 3 years, and crack on with the mission full time. If you are shrewd with your money, you can most likely do the whole thing at a reasonable cost, in comparison to some of the larger integrated schools.
I have said this many times on these forums, but after all the hard work, blood sweat and tears, if you heart is in flying, when you land your first professional flying job, it will be worth it and you will never work again
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All the best with your quest.