Pilot qualification

Yes, you technically only need GCSE's to become a pilot to apply to most flying schools. The challenge more is the tests to get into the flight school. Have a look at L3 Aviation Academy or Oxford Aviation Academy's (The two major flight schools in the UK) websites to find out a bit more!

Is it advantageous for a candidate to possess A-Level certificates?

It can be, yes, just like any other job. But if you do have other appropriate qualifications, they will be happy to interview you. I recently had a careers interview at L3 Aviation Academy and they said that it was fairly easy to get into the assessment days even if you don't have A-levels and have other qualifications, It's just on the assessment days they are looking for the right person, and they will interview you like you were applying for a job with an airline. And the tests are very difficult. If you want to find out more, go to a flight school open day, they are free and usually happen quite regularly (at L3 they are monthly). There they will tell you everything you need to know!

1. After completing, you have the main licences you need. However you do need to get a type rateing (a licence to fly a particular type of aircraft e.g. A320, 737) but that is done when you get a job with an airline.
2. Yes, it is very expensive. Around £100,000. You can go around banks though and see what loans they can offer you. Parents are known to morgage houses to pay for it. However if you get a job with a particular airline you can get up to £70,000 back over some time on top of your wages.
3. No it does not. However during the process you can get a degree out of it if you do a bit more work, this means that you can also qualify for a student loan.
4. Your eyesight needs to be ok. As long a you can pass a medical and you aren't colour blind you will be fine.

Good Luck!