Because if they find out you're lying (or, indeed, suspect you're lying) and deny you entry, you'll forevermore have trouble getting in to the UK. Place I work

Because if they find out you're lying (or, indeed, suspect you're lying) and deny you entry, you'll forevermore have trouble getting in to the UK. Place I worked at during the late 90's had the UK immigration folks turn up and tell a US citizen on a tourist visa to get out of the country. He was working illegally on a tourist visa while he was waiting for his work visa to be approved. Suffice to say the work visa never got approved after that.