You may wish to look at easyhotels the easyjet branch of hotels, for a bog standard room they are pretty cheap they are all over London. The easyhotel in the Barbican area of the city is relatively speaking close to the o2 arena. I'm staying in another one in West London, cost me £44.00. Also, although a 40 minuteish tube ride I do recommend Huttons Hotel in Pimlico, West London fairly cheap
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