The obvious point to make is that your hotel isn't in London - it's in Hertfordshire some 15 miles from the centre of London. It would take you two hours to get there by train from Stansted. By train it's 30 minutes to get to London and costs £10 per day return. On top of that you have another £6 for the cost of getting around London.

Staying there is a false economy and I'd cancel and find somewhere IN London.