Not the kind of thing anyone on this forum would ever say but you might be best advised buying a cheap bike off ebay - you can pick serviceable secondhand bikes up for £10 to £20. Id estimate about a 19 - 20 inch frame mountain bike would suit, hybrid bikes are more expensive even second hand. Then if you get hooked you can investigate buying something better - as hlab observed decathalon are well regarded.
My only issue with this is if I get a real dud, it could completely put me off cycling.
Which I guess is the same issue with buying a cheapy anyway - hmmmm