I'd opt for the hybrid bike myself. Possibly something towards the lower end of the price sectrum- there are some good deals on Carreras at
Halfords at the moment. Get some miles in over the next few months then maybe look to treat yourself in the new year. If you get a £200 hybrid and you have another £600 or so in Jan/Feb then an Allez or Secteur, or one of the Giant Defys might be in order.
Also, you'll probably find that right now your local cycling club are doing huge distances- if you in spring they'll be doing less, and you could build up the mileage with them.
The biggest thing for weight loss in the western wrld is diet though.