Mark
No one can make you claim State Benefits, you may need to due to financial needs but no one can make you claim UC.
If you are found Fit for Work then you can immediately appeal the Decision and receive the ESA Assessment rate until your appeal is heard.
You are correct, if you claim UC then you cannot return to ESA even if your appeal is successful, however, any award made by the Tribunal will be reflected in your UC.
Gordon
Nothing on this board constitutes legal advice - always consult a professional about specific problems