STA is having a great sale for the flights all over the world.
I think these are really good prices:
New York (Return) from £259 (under 26), from £269 (over 26)
Bangkok (Return) from £399 (under 26), from £415 (over 26)
Los Angeles (Return) from £365 (under 26), from £394 (over 26)
Sydney (Return) from £685
I believe it is a great deal considering those small differences between under and over 26. Usually they are more significant.
There are more deals for Auckland, Melbourne, Nairobi, Sydney, Rio.
I suggest if you want to book a flight with them, go to their office - in that way they won't charge you any additional fees for booking over the phone.
The sale has started on 26/12 so I believe some good deals are still there.