It doesn't matter which exam board it is, if you got the answer right then they'll give you the marks for it. You weren't very specific with your wording, for example "There is a constant supply if blood from the body which has little oxygen and a lot of carbon dioxide" is a bit long winded, when you could've just simply said blood in the pulmonary arteries is deoxygenated (which is not true, it will still contain some oxygen, but at A-level it's acceptable) contains little oxygen and excess CO2. Also, it would probably be better mentioning that O2 diffuses into the capillaries of the lungs from the alveoli, instead of the haemoglobin.
You'd definitely get one mark for mentioning that the O2 and CO2 diffuse along a concentration gradient. Not sure about the other though.