there's a designer outlet outside London in Bicester, Oxford. There are trains and coach excursions from Central London that can take you there. Here is the link of the outlet:
But it will take you an hour from London Marylebone on the Chiltern Railway to Bicester North (some 20 quids for a return ticket), and then take a taxi van (similar to our Green Maxicab in Hong Kong; this costs you one quid) to get to the outlet.
A very good Italian restaurant there - Carluccio if you wish to get some energy!