I as able to use the dates on my passport. Do you have any frequent flyer miles with any airlines? How did you purchase your tickets? Credit card, check, bank check card? All of those should give you an idea of when you bought the tickets. Did you use any of your credit cards while in the UK? Could use the dates from those statements to help pin down the date. How about the person you were visiting, help with remembering dates? Emails or IM logs if you spoken about the trips in either of those? Hope you get it worked out.