OK where to start.
First things first - medicals.
You need to sit a Class 3 Pilots Medical (IIRC) which costs a few hundred pounds and needs to be done by a certified CAA doctor.
IF you pass this, you can continue. If you dont....
The levels of aviation licensing is as follows:
PPL Private Pilots License
CPL Commercial Pilots License
ATPL Air Transport Pilots License
As far as i can remember you need to have acheived ATPL before you can officially get a job for a commercial airliner.
In between these levels, there are lots of different "ratings" you HAVE to acheive:
Multi Engine Rating
Night Rating
IFR (instrument flight rules)
You also then have to sit all the examinations for all of the respective steps - airlaw, air nav, principles of flight etc - all of which cost money.
The best bet is to contact airlines or cargo companies to see what the requirements are.
There are two main avenues you can go down:
1) Sponsership. This is where you get sponsered through all the training levels by an airline. The Pro of this is that they pay the extortionate costs required to get the ratings, but the big Con of it is that once qualified, you are generally tied into contract with that airline for a pre-determined length of time - typically 5-10 years. This will potentially hinder your career development as you may be able to get a better paid job with a different firm, but not be able to as you are signed into contract with your sponsor.
2) Pay for it yourself. This is a risky way to go unless you are a) loaded b) prepared to take any old job at each step of the way to gain the work experience at each stage. Its hard to say typical costs of getting trainined to ATPL with all the required ratings, but its somewhere in the region of £50 - £70,000. AND EVEN THEN, once you are qualified, a lot of airlines wont take you - as they want you to be trained THEIR way. This happened to my instructor whilst i was training to be a cadet instructor.. he had just acheived his ATPL and applied for a job at monarch. They turned him down on the basis that he was "too qualified for the job" - which basically means they wanted to train the successful applicant their way, and not have someone who had already been trained.
He had to settle for flying antisocial hours for a no frills airline for many years before he had the relevant experience for other "main stream" airlines to consider him.
Of course, the whole issue depends on what exactly you want to do in aviation. If you want to fly recreational flights over the lake district for example, you only need a CPL. IF you are only flying during VFR (visual flight rules) and during day light hours - you wont need the Night Rating and IFR ratings. If the aircraft you fly only has the one engine - you can save about £5-£10,000 on your multi-engine ratings.
You say that you wont go for your licenses until you definately know you can get a job as a pilot. Im afraid, the only ways to get a job as a pilot are the two ways mentioned ahead. Unless you are a graduate, or an outstanding candidate - the airlines will be reluctant to take you on board as a sponsoree. Which leaves you with the option of training, and THEN applying for jobs.
Consider a career development loan from the banks.... thats what my best friend who i was training with the RAF did.
IF you have any particular questions you would like answering, please feel free to PM me and ill ask my colleagues from RAF (many of whom are qualifed CAA pilots too) for the answers.
Joe.