Our family will be in London mid-August. We want to visit the usual sights -- Houses of Parliament, Tower of London, Churchill War Rooms, Westminster Abbey, Buckingham Palace -- for those sights requiring tickets, do you recommend purchasing on-line ahead of time? Will it assure us of getting in and save us much time waiting in line? Or is it easier/OK to just show up and hope for the best. CLearly, if there is any chance of not getting in or having to wait hours in line we'll opt to purchase tickets in advance.