International Trip
 

(I have a passport but I've never used it...)

What is involved in terms of passports and visas and such to get from the US to the UK ? How much advance planning is required?

Tom,

Its about as simple as it gets, and no advanced planning is required. You do not need a visa, and if you're just visiting, all you technically need is a return ticket, although I've never heard of UK immigrations asking for one. Just remember to bring your passport as you will need it to get one the plane heading over, clear immigration once there, etc.

The one and only time I've ever been asked for a return ticket was at LHR.

even coming from Switzerland I was asked about every third time for my return ticket at LHR.

And I guess the US passport must be valid (not expiring) for another 6 months after the entry date to the UK?