How do I book cheap flights?

The website says a flight from Glasgow-Alicante is 19 pounds, but when I tried to book it, the flights listed were much more expensive. I've tried to sign up for the newsletter, but the confirmation email has nothing in it. please help! these were the requirements (which I followed) for the good deal:
Booking Period: Travel Period: Applicable Days: Flight must be purchased:

Mon 1 Mar 10 - Thu 4 Mar 10

Mon 15 Mar 10 - Fri 30 Apr 10

Mon - Sun (Subject to availability)

14 Days

Hello,

Firstly I don't know the answer to your newsletter problem, but have heard a few people mention that they have had problems trying to sign up for it lately!

Secondly, re your Glasgow - Alicante £19 flight.

From what you say it appears that all of the £19 flights have gone.

In general, the prices will start off at £19 (normally not for ALL Glasgow - Alicante flights, usually a small number) and as these flights get taken and the aircraft begins to fill up, the prices will usually start to steadily increase.

The second last piece that you have put in your posting covers them:

"Mon - Sun (Subject to availability)"

Subject to availability is exactly what it means I'm afraid.

Maybe not the answer you were looking for but this is the most likely reason.