Hi Daisycat,
The only ESA claimants who do not have to undertake WFIs, if asked to do so, are those in the Support Group or people who have an active ESA claim but are not receiving any payment. That means that people still in the assessment phase can, and are, obliged to undertake WFIs. Therefore, I'm afraid that you cannot draw any conclusions from this.
It might be possible to have the WFI carried out over the phone, although this is entirely at the discretion of the advisor. They do have to take into account any special needs that you have as a result of your condition. You can try phoning the number on the appointment letter and speaking to the advisor about this.
Nothing on this board constitutes legal advice - always consult a professional about specific problems 
