Option 1: Go on the train. From Olney there are 2 main options to reach London - via Milton Keynes Station into Euston, London or go via Bedford station which takes you into St.Pancras.
The quickest route from Milton Keynes takes just over 40 minutes (make sure you choose a direct train without stops). Book ahead for the best deals.
There is longterm parking at both stations. www.nationalrail.co.uk
Option 2: Go on the coach from Milton Keynes coachway. At the weekends, this should take about an hour and 45 minutes. There is parking at the coachway, or bus links there. Again, booking ahead costs less. www.nationalexpress.com
Option 3: Driving. Doesn't have to be hideous! The congestion charge isn't operated at weekends. On Sundays, some parking metres aren't in operation, so driving to the Natural History Museum, the V&A or the Science Museum for example becomes realtively easy.