Can you prove at any point that they lied? Does any of their literature explicitly state this? Or does it just say "under certain circumstances"?
Bob Jones has some in-depth stuff up on this site and is an ex-tax inspector so really knows his stuff.
This is probably the most relevant part to you:
Expenses guide
In a nutshell, most of their employees move from site to site whilst 'employed' by them, and there is reasonable expectation that you will do the same as that's what you've signed up for.
However, you have now said that you will be leaving so won't have another site. Therefore your current workplace has become permanent.
Not much you can do about it.