As far as I've heard and know the youth cards are really worth the effort to get them. Unless you find a specific discount that the card allows you (I'm not sure if you need it for the blue ticket or not). I know you can get a decent amount of discounts through companies, hotels, travel groups, etc that have an agreement with STA. Here's a list of the local discounts for the youth (under 26 card): http://www.statravel.com/cps/rde/xch...hs.xsl/317.htm
With the blue ticket I'm pretty sure the major advantage is that you can change you travel dates for a minimum fee and can stay in a country for up to a year (as far as when you departure flight is). So if you were planning to go to Japan for a month but you ended up finding a job and a cheap little apartment to stay at, you could easily call STA and have them extend your flight. I'm guessing if you're going to be on a short trip or one were your set in the dates of travel, it may not be worth it. Also if your traveling to a country were public transport is slow and your covering a large amount of ground on your trip, it may be in your best interest to get the blue ticket just in case a bus or train is late or broken down. I've never used the ticket myself but I was considering it for when I leave in June for a year long trip. From what the site says the ticket is good in 90 countries - it works just like a regular ticket from what I can tell....you just have more flexibility with the dates.
Since you live in England I'd stay just use Ryanair and the other discount airlines there!