I know it's so difficult HB - seeing blood in the loo is just not right. I have had many sigmoidoscopy and they are not agonising - they are uncomfortable but sometimes sedation is possible if you ask. I always have sedation as my bum is very narrowed as I had a bag 39 yrs ago. Bright red blood suggest fresh blood like haemorrhoids or split - coffee grain colour / consistency tends to be more worrying.
I hoe you don't have to wait 12 weeks - could you phone and explain you will accept cancellation ? Health anxiety is awful but surely if GP was concerned you would have been fast tracked?
All the best HB xxxxxxx