Rolls royce do a scheme for 16 to 19 year olds where you get a chance at work experience but I think it only lasts a few weeks. If any company does a scheme for pre uni students it will be similar to the rolls royce one, I doubt any will do a proper 12 month work placement as you haven't actually studied any engineering yet and as a result wont really be able to take up such work. However in saying that there is no reason why you couldn't email various companies and ask if they have any positions for people like yourself. You never know they may need a part timer to do odd end jobs, it wont be proper engineering work but the experience will be invaluable and having that on your cv before you even start uni and apply for proper placements will give you a great advantage.
Also instead of looking on websites for placements and jobs, try enquiring directly to the site you want to work at because like I said most companies wont offer pre uni placements and if you contact the main office they will just tell you they don't have such pre uni schemes or dont offer odd end jobs. However if you phone or visit the site directly then you could possibly talk to one of the managers who may be more inclined find you some sort of work in the site. I managed to get a work placement at a munitions company despite them not actually offering placements because I went directly to the site and spoke to the manager who just basically employed me on a 4 month temporary contract for the summer.