Like others, I would agree that to get the best experience of London in a weekend, you should really be staying in central London.

But, if you can't cancel your reservations, its not too bad. Your hotel isn't far from Elstree & Borhamwood station, from which you can get to St. Pancras in about 25 minutes.

However, it is quite a long and inconvenient journey by rail to this hotel from Stansted airport. Instead, your best option could be to take the National Express 737 coach from Stansted Airport to Hatfield. At Hatfield, change to local bus 615 (from stop "D") which stops next to your hotel (Oaklands College stop). You can book tickets and check journey times here: http://www.nationalexpress.com/ - this would be cheaper and quicker than travelling by train via London.

For convenience, I think you'd be better off renting the car and driving to your hotel. There is parking at Elstree & Borhamwood station for £5.50/day and you can take the train into central London from there.