The official title of the kind of work permit that you will need to obtain for holiday work inside the UK is called a UK Working Holiday Visa. According to this website, you can obtain a UK working holiday visa for up to two years if your sole purpose for coming to the UK is not strictly for work. You can perform part-time or casual work; however, the age limit upon this type of visa is 17 to 30 years old. For more information on how to get a new holiday work permit for the UK, visit the website, which also affords you information on the British embassies and consulate's offices and offers applications that can be downloaded for the purpose of obtaining work permits in the UK.