When I sat my NATS selection tests (1995), before we started we had to sign a bit of paper which if memory serves said:
Please select the following as applicable and sign below:
A) I understand that I will be employed by NATS as a mobile grade and agree to be posted wherever it suits the company.
B) I do not agree to employed as a mobile grade and wish to withdraw my application 
Signed..................................
Not word for word but you get my drift! Don't know if they still do this or not.