Thats not low. Average UK salary is about 15-17k apparently. Unless you have some sort of super duper skill your lucky to even get min wage.
average (median) is something like 25-26k, average (mean) is something like 33k (although a few mega high earners skew this figure)
that said, an interpreter salary is still quite good, and bear in mind that, while it requires a specific skill (being bilingual) there isn't going to be much room to advance, what is the promotion to being an interpreter? and the best salaries are going to come from advancing and promotions