WCA whilst contracted 8 hours

I have just done my identity appointment with UC and I am aware that I will need to send fit notes to trigger the WCA Assessment and form being sent ( not sure how long I have to send fit notes for so if someone can let me know that will be helpful ). I also work part time contracted 8 hours so I am not eligible for sick pay and I can’t really afford to actually go off sick. I know you can still work and send in fit notes but what would I have to ask my GP? Would i still need to send the fit notes to work aswell as UC?