Hi Max,
Until you have been allocated to a long-term group, payment of ESA is dependent on the time span being covered by a fitnote (sick note/GP certificate). It does not matter exactly what this says, even if it indicated that you might be able to undertake light duties (for example).
It is up to the individual GP to assure his/herself that the patient meets the criteria for issuing a fitnote.
If you cannot get such a certificate your only option for money would be to claim JSA.
Nothing on this board constitutes legal advice - always consult a professional about specific problems