When things change the surest way to identify a cause is to identify all the conditions which obtained before the change and then look for new conditions which have emerged since the beginning of the change. By this method the rise in youth unemployment in the UK is simply explained: it is the result of the great increase in immigration to the UK. 
The rise in youth unemployment started in 2004, long before the current economic woes began in 2007. This coincided with the removal of restrictions on the movement of workers from the new EU states (the Czech Republic, Hungary, Latvia, Lithuania, Estonia, Poland, Slovakia, Slovenia, Malta and Cyprus). Hence, the rise cannot be blamed on the poor state of the economy.
None of the other explanations offered for the rise in youth unemployment are plausible. The British education system has not changed since 2004; the attitude of young people as a group towards work cannot suddenly have altered; contrary to popular myth, most jobs are low-skilled or unskilled so a skill shortage does not come into it for the vast majority of jobs taken by immigrants; where skills are needed it is wildly improbable that the skills available dropped off a cliff at just the time immigration rose massively.
If employers could no longer import cheap labour they would be forced to employ Britons (including young Britons), offshore their business, or cease to trade. As most British economic activity has of necessity to take place in Britain, either because the goods can only be made in Britain (for example, Scotch whisky) or the service has to be provided in Britain because it is of a nature which makes this necessary (for example, virtually all public services and retail, transport and energy businesses), offshoring is not an option for the vast majority of British employers.
As for ceasing to trade, it is unlikely that there would be a large amount of that occurring because the wages paid to adult immigrant workers would be at least sufficient to cover the wages of young British workers. As for the idea that young Britons cannot do most of the jobs that immigrants are doing, this can be easily seen to be a nonsense. In areas of Britain where there are not large numbers of immigrants, the jobs which are supposedly beneath Britons are done by Britons. Moreover, we know that before 2004 British youngsters were being employed in the jobs now being done by immigrants.
For skilled jobs, there are huge numbers of unemployed British graduates who either cannot get jobs at all, or who are forced to do jobs for want of anything better which do not require a degree. 
The claims by British employers that they are employing foreign workers because they cannot find suitable people is hard to credit. Even if there were a problem with the attitude of young Britons, for which I see no evidence in general, it would not explain why older workers with a good work history are being overlooked. In particular, it is implausible that foreign workers are better equipped for jobs dealing with the public, because many foreigners employed in such jobs have inadequate English and a lack of knowledge of British culture.
It is important to understand that many jobs in Britain are effectively placed out of reach of Britons of any age. Foreign gangmasters are widely used and frequently only recruit people of their own nationality. British employers find foreign workers are cheaper to employ, easier to control and less difficult to lay off. A substantial proportion of the jobs, especially the low and unskilled, are going to illegal immigrants who are even more vulnerable to demands from employers. Foreign companies in Britain often bring in their own people. 
When foreign workers gain a foothold in an area of business they recommend people they know for jobs. Being foreign, the people they recommend will normally be other foreigners, especially those of their own nationality. It does not take long for a place of work to become largely or wholly foreign staffed with this type of recruitment . There are also agencies which only supply foreign workers. Public service organisations and large companies often use such agencies. 
Sometimes the employer has employment practices which effectively exclude Britons, for example, Pret a Manger use, as part of their selection process, a vote, by the staff of a shop where a potential trainee has had a trial, as to whether the trialist should be taken on. It does not take too much imagination to suspect that foreign workers will vote for other foreign workers, especially from their own country, if there is a choice between them and a Briton. 
The only way the young in Britain will be able to get jobs is by regaining control over Britain’s borders so that mass immigration can be stopped. To do that Britain would have to leave the EU or come to an arrangement with the EU which prevented free movement of labour from the European Economic Area (EEA) to Britain. It would also need a government willing to cancel all other forms of mass immigration from outside the EEA such as family reunification and reinstate the primary purpose rule governing those coming to Britain to marry.
The brutal truth is that if mass immigration is not ended the situation will continue as it is and quite probably get worse as the Euro crisis worsens. It is self-evident that if millions of experienced workers, willing to work for low wages, are imported into a country the size of Britain, they will displace the native workers generally and the young and inexperienced native person in particular.