If your doing BTEC Extended Diploma in Applied Science Level 3, then yes Distinctions are quite hard, I mainly use A Level websites in compliance to my Units for example one the first units you will do I think is Perceptions of Science. Which invovles mainly theory rather than practical, but it requires critical and ethical thinking to answer questions but it isn't hard to get distinctions but other units such as Physiology oF The Human Body Systems, you should use the website biology mad a level and it will explain everything in detail.
If you are sturggling with any current units just private message me and I will be gladly to help you