Have just completed the final pre-purchases online - tickets for the Heathrow Express, which saves me 10 pounds - money better spent elsewhere.  Have also bought an Oyster card to get around on the tube - much cheaper than buying individual tickets.  A good website to do this is visitbritain.com.  It's amazing we can do this ourselves on the web now - it really reduces and in many cases removes the need for a travel agent - my airline ticket was online, as was the train tickets, internal flight, accomodation bookings, theatre tickets, tube ticket, tour etc etc.  Not long to go now - and I will be on holidays - I wish it would get here quick!