First question will be what surfaces you intend to ride on?
Unless you are planning big distances and pushing it, cycling isn't great for losing weight.
I'd do food and walking first, unless by big you mean massive Arnold type muscles.
I wouldn't normally recommend a bike like this as it is very heavy, and the forks have little damping - those big tyres should support you well though.http://www.halfords.com/cycling/bikes/m ... ntain-bike
Can get a further 10% off with membership to something I cant remember right now lol, British Cycling club or something. Unfortunately there is little else that is any good for this cash. Decathlon do a lighter bike for £240 ish, but much the same parts.