I had more than 5 job offers at St1 level

Very urgent help !!
I had more than 5 job offers at St1 level
I didn’t know the exact procedures regarding the COS as it’s my first job in the NHS
I sent one job the filled documents but stated clearly in my email that I want full details about Rota and on call supplement and time limit to start the job before issuing my COS as covid 19 situation in my country is still very aggressive with no leave permits
Now after sending the email they didn’t answer my questions and sent me the COS within an hour !
Now I have much better offers and all data I asked is clear
How I can cancel my COS for this job
This photo is from Uk government website
A lot have said it is not advisable to cancel it ( black listed and trust can make a complain against you )
Your advice please