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 opeation, so driving to the Natural History Museum, the V&A or the Science Museum for example becomes realtively easy.