Hi,
The first year of the programme is purely undergrad econ. If you have a strong background there, it's just an additional 20k Pounds.
If your Econ background isn't that strong, you should go for the Two-Year programme. It seems like admissions are on the basis of academic strength and quantitative background. Many students hold bachelors in Econ or related subjects. So you have chances, depending on your performance in the undergrad.