There's a whole series of complex things to consider.
First, the impression I got was that the bulk of the UK immigrants weren't coming from places like Spain or Greece, but rather Poland, India, and Nigeria. Correct me if I'm wrong.
Second, I was also under the impression that the sectors migrants target are sectors where there is an actual shortage of applicants - like truck drivers, plumbers, electricians etc. It seems that migrants are much more professional overall and not doing low-wage menial labor.
Third, the UK recently made it more expensive to hire agency workers, which were a way into the labor force for youth.
Fourth, the UK is making education more expensive, while education in places like Poland is both cheap and high quality. There are simply more migrants with needed skills than young people with needed skills.
Fifth, the UK certainly does have an aging population problem. This is also related to youth unemployment, as older workers who should be retiring are doing the jobs that younger workers used to do. So if you're going to blame somebody to taking young people's jobs, blame the elderly.
http://www.parliament.uk/business/publications/research/key-issues-for-the-new-parliament/value-for-money-in-public-services/the-ageing-population/
http://www.theguardian.com/business/economics-blog/2013/mar/13/workers-aged-65-older-increasing