The Japan: Youth Exchange scheme allows Japanese nationals between the ages of 18 and 25 to come to the UK for an extended holiday for up to a total of 12 months, which can not be extended. You can take up employment but this should only be incidental to your holiday. There is a quota of 400 participants per year. Japanese nationals can only take part in this scheme once in their lifetime.
You can come to the UK under this scheme if you meet the following requirements:
If you don't intend to work or don't have a realistic chance of finding the type of work you want to do, you are not likely to qualify. You will also need to apply for entry clearance at the British Embassy in Tokyo.
Once you arrive in the UK you can take up full-time work for only half of your stay. Full-time work counts as any employment of more than 25 hours per week. You can work part-time during more than half of your stay as long as you intend to take a holiday as well. You should not intend to work during your entire stay, but can take up employment with a voluntary organisation or charity as long as you wish.