This time last year I managed to score a job at Asda. I'm 18 too, and at the time It was the summer before I started uni, and I am so glad I found this job, not because of the monotonous tasks, etc, but the flexibility; its great! I now work seasonally in a store in Yorkshire, mainly summer, christmas and easter holidays. All I do is say that I want to come back to work, even the odd weekend, they'll take me on, then when I want to leave, I just tell the personnel dept.
Right now, many of my school mates are stranded, looking for jobs which they crucially need to survive the next uni year, while I still have mine intact.
In fact, I can transfer my job to where I live the rest of the time (North East), or If I decide to move to America for a year, I can transfer my job to their Walmarts easily.
Definately recommend that you apply there, jobs definately not great, but its the flexibility that counts.
I'm also looking for a second job, just for the summer