I was just looking at internships with the United Nations and one of their requirements state:
1. You are currently enrolled in a Master or PhD (or similar programme) and will be during the entire internship, or You are pursuing studies in a country where higher education is not divided into undergraduate and graduate stages, and you are currently enrolled in your fifth year (or higher) at a university or equivalent institution towards the completion of a degree.
So how can a person doing a 3 year degree followed by a 1 year masters apply in the UK apply?
If you mean how can you, well you satisfy the first condition of being enrolled in a Masters program. The 2nd option is "or", not "and" be studying in a country which makes you graduate with masters level knowledge.