belleh, it sounds a tad expensive to invest something like 10.000 pounds into a PPL just to see if you like flying for the airlines.
First, it is much cheaper to get to know a few airline pilots and chat to them, ask them what their typical day looks like, what their career prospects are, what they find most satisfying/rewarding in their work and what they hate about their jobs.
Second, the world of PPL style flying is far, far removed from the world of ATPL-style flying. Yes, the laws of aerodynamics are the same and we sometimes share the same airspace and airports, but that's about all there is to compare.
A PPL pilot, in a lot of cases, is an actual pilot. He or she is hand flying at relatively low altitude, in the weather, navigating by relatively simple means (lots of GA aircraft do not have an autopilot coupled with GPS, let alone an FMS) with just one or two passengers on board, flying for fun at convenient times, not really to a schedule. Flight preparation, refueling, catering (sandwiches and a thermos) are all things that you do yourself.
Airline pilots are more flight managers than anything else. They have to get up at ungodly hours to operate a flight to a schedule that's set with a precision of mere minutes. The taxying and take-off is done by hand but as soon as practically possible, the autopilot takes over, is coupled to the Flight Management System, and takes the aircraft to an altitude far above the weather. The flight itself is spent checking systems, communicating with ATC, reprogramming the FMS and reading books/magazines/papers/whatever. I've even heard of airline pilots who are using the FMS to play games with each other! On the ground, the pilots are helped by a professional crew of refuelers, caterers, dispatchers, cleaners and so forth, but all that work needs to be checked and corrected as necessary. This is the most hectic period of the flight - even assuming everything goes according to plan.
So. If an airline career is something that you think you want, get as much knowledge in as you can. Talk to schools, talk to pilots. Get a class 1 medical, get a loan and go for it. You'll get your PPL as part of your training anyway. But don't expect that a PPL will give you great insight into what an airline career is about.
(Disclaimer: I'm a GA pilot with no personal experience of commercial operations, but I've heard a lot about them.)