Depends on how long you were out of UK if it was for many months then you might have problems. If it was for few days you might get away with giving them approximate dates. Usually Passport have all this information but I had couple of miss stamps also on my passport and was able to get it with Airline reservations and tickets. I am not sure about the Travel Documents. If you make a point of keeping a copy of every document you submit to the Home office, you won't have problem if they dont return your documents. Home office is famous for even misplacing passports, I just dont trust them with any documents.
Can you find out with the carier you took like airlines or something. if you booked it with Credit Card, you can also find the date you paid or find out from travel agent if you make arrangemnets with Travel Agents.
All this information is shot in the dark but if I know the circumstances, I would have given you more specific info.