I went for ten days during my freshman year of high school, over spring break. 45 of us were picked for the trip based on our grades, etc. in school.
And it was the best trip I've ever been on. We learned so much and saw so many places. I met so many nice people and I want to go back as soon as I can.
We saw London, York, Bath, Salisbury, Stratford-upon-Avon... and a bunch of other places whose names I forget. :P It was absolutely fantastic.
Hopefully I can study abroad there when I go to college, because I love it there so much.