When im on an exercise bike at the gym i sometimes find my throat starts to feel smaller & i find it harder to breath. why is this?

EIA: Exercise induced asthma is exactly what it says. In other words, physical activity brings about bronchoconstriction & shortness of breath. A short acting beta agonist such as albuterol is used in the states to prevent this from happening. You'll need to ask your gp what's available in great britain. That's assuming that you have eia. Bottom line: go see your family doc for evaluation & treatment.