Thr trouble with clients declaring that a contractor is caught, they are also tacitly admitting that the contractor is indeed a full time employee, as per the comments of Judge Burton in the IR35 Judicial Review nearly 20 years ago.That means the contractor is due paid holidays and all the other rights that go with that position.It is debatable if the rights are removed even if they push the contractor into working for an umbrella company either.
Of course as most contractors are in it for the freedoms contracting offers and not the tax benefits most contractors are not going to enforce this and have decided that walking is preferable.