Although it's the trend to go ultra-wide (10mm etc), you don't need to for landscapes; these ultra-wise lenses add distortion to horizons, making them arc across the frame rathe than go in a straight line, and sometime shots can just look a bit too crazy.
Something like a Sigma 15-30mm might be a good buy, but I'm not sure you'd get one for under £250 unless you really hunted around on the used market.
Another good lens optically is the Tamron 17-50mm f/2.8; superb lens that you can pick up new for about £260 these days.
Have a look at
www.photozone.de for lens reviews - this might help you shortlist some good lenses.
Both these come in CAF fit.
My opinion is that for landscapes you'll be using a tripod for extra security more than anything else, so you can afford to go for slightly slower (max aperture) lenses. he Tamron is good because it's f/2.8 throughout but for landscape work you'll generally be shooting around f/8-16 anyway, negating the need for a really fast lens.
As TLR has said, you don't always have to shoot landscapes with wide-angle lenses neither. Some of the best - David Noton, Joe Cornish - use longer focal lengths to compress perspective and create more graphic elements that don't rely on a major foreground interest that you'll generally need with a 10mm lens.
Do your own thing; the Pyrenees are magnificent and regardless of the lens you use, there will always be something brilliant to shoot.