I am not sure what the state of their computer system is these days. A few years ago they had no idea who was coming in and out of the country and when, but I think they've upgraded by now so if you apply for a fiance visa or whatever, they will be able to see when you came in and out of the country, regardless of the stapm in your passport. This is what I would do - call the Home Office ASAP and explain. Write down verbatim whatever they say (so you have a reliable record of it) and act on what they say. That way if you are asked about it later, you can say it was an honest mistake, you contacted the Home Office and followed their instructions. I suggest you be prepared to fork out whatever it costs to change your ticket though.