It takes me an hour on the train into london, then just under an hours walk from waterloo to where im working at the moment.

If you want to exercise you have 2 choices, either moderate exercise for a long time period (like an hour walk each way to work) or hard work for a smaller amount of time.

Ah hours walk each way would see you consuming about 600 calories or so.
Cutting out snacks would probably save you another 600 or so quite easily.

So that everyday and you will lose about 2 pounds in weight a week, or a stone every 2 months.

Up to you mate, no one else gives a fuck if you want to stay fat or not, so if you cant be bothered to do anything about it no one else is going to.