Earlier quoted context omitted.
Then change your password to something that you don't actually know. Generate a random password before your trip and print out a few copies. Take one with you and secure the remaining ones. Before you fly through the UK, change the password on your computer to the random one and destroy the printout. When you get back home use the remaining printouts to change your password back and then destroy them.
These sorts of techniques make great comments, but are nearly useless in reality. First of all, even if you're being truthful, saying "I don't know my password" will likely not be believed. Explaining the process you went through to safeguard your password will merely elicit suspicion by a border agent. Why would you go through such pains if you had nothing to hide? And once they are suspicious of you, you will almos…
Obviously the best approach is to just not have your laptop.