Buy some prawns. Leave them on the side for a couple of days before eating them and the weight will drop off!
Seriously, though, introduce simple exercise into your life. Take the stairs instead of the lift / escalator; don't get the tube, walk it where possible (often easier on the way IN to work than on the way home); buy the cheapest bike you can and cycle to work if you have the facilities to shower / change
Change your diet SLOWLY: eat porridge for breakfast to cut out morning snacking on crap food; ditch the lattes, put £2 a day in a bike fund instead.
Long term habits are acquired slowly.
Once the bike is built, you'll be in better shape to enjoy it. Plenty of decent riding within a few miles of Crystal Palace.