Two weeks sounds great for that ride.
Living in Portugal, I know Spain well and have toured around that area, from Pamplona southbound, but not as far as Barcelona. Roads are good, but beware: any two towns 20km apart in a straight line will take you two hours hiking in the mountains, but the same or more on the road, as you have to go around the mountains.
I'd say Spring or late Summer are the best times to go. Being a brit, you'd suffer from the heat during Summer, plus the amount of tourists will largely increase prices and traffic, including loads of large parties in loads of large buses. Weather may vary in Spring and late Summer so be prepared for the odd heavy shower.
The scenery is unique. Camping is a good option, although sudden weather variations may be a problem if your gear is not tip top. If you book ahead, houses in the mountains are available at a reasonable price. I'd go for camping.
Apart from the mountains, briefly visit the main attractions, but try going off the beaten track. The Iberian Peninsula is full of pleasant surprises for those who dare.
Try the smallest of towns along the way, but away from the main roads. If you dabble some Spanish, perfect. If not, do learn the most basic stuff. It will make all the diference and turn a pleasant trip to a memorable journey. I know brits *usually* stick to touristy places with at least one pub around, but I'm assuming a biker will be a bit smarter:-)
I think the XR250 will do just fine, and you'll find a lot of off-roading available. Just make sure you're not going where you're not wanted. If you want to mail me at my home address ( email@example.com
) I'll find a magazine I have somewhere with the contact of a company that organizes off-roading in the Pirineus. They might give you some free advice, even if you do not enroll in their programs, I assume.
Do reserve time for two things if you value them: time to enjoy the mountains, its silence and its wildlife, and time to enjoy Barcelona. When you reach Barcelona, you'll wish you had another two weeks to stay :-)
Hope this helps. Do mail me for more specific hints or that off-roading contact.
Have a great time!