You would need to look at the small print of your contract. If it doesn't explicitly state that if at any future time the value of the property drops and the LTV increase above 80% then I believe you should be in a good position.
I don't have any written contracts(other than the original variable rate contracts). Both Banks switched me to trackers when I started the process of changing my mortgages over to Halifax. It was all verbal with very little written done, which seems to have strenghtened my position.
Whatever your contract states I would resist any attempt by the bank to get you off the tracker when the interest only period ends. If they do try to get you onto standard rates then I would contact the Finacial Regulator.