Bicycle commuting in Britain is not healthy. I tried it once and my throat was sore for the whole day. Cycling is basically exercising and greatly exchanges the breathing volume. The exhaust fumes, tire dust and brake dust from cars and trucks is simply too much in British cities. Motorcycles and mopeds are even worse. These gasses are toxic make you feel light headed. The particulates damage your nerve system and greatly increase your chance on Alzheimer.

Not saying a car is much healthier either, but a) with a good modern car you have a great air filters
b) your breathing volume is much lower.

The train is still the best option for me. No car congestion and generally much cleaner air.

I would only consider commuting on a bicycle if you could constantly ride on country roads or independent cycle paths far away from car congestion, but the simple fact is you can't.