John,
Just worked out, as a PAX, I have used LHR (in or out) nearly ninety (90) times over the last couple of years, the last time being last wed and the next time will be on thurs. During that time I have never had cause for complaint. I have to say, in my experience, that ALL the boys and girls there who make the place work are very professional, courteous and helpful. You have no worries, you'll be fine...
Enjoy your holiday/vacation!
Foxile