Passport Question

When you land in London, since you will be staying there, you will go through immigration to enter the UK, customs is separate, but a non issue, just follow the green lane out (nothing to declare) unless you have something to declare. To head to Frankfurt, you will go through an exit passport check in London, then on arrival in Frankfurt, you will need to go through immigration again to enter the Schengen zone.

how quick is "somewhat quick transfer"?

Frankfurt is not a small airport.

I'm confused on your route. You say you're starting in London and then going to Frankfurt to take your tour. But you then speak of transferring in Frankfurt; from your first paragraph, I pictured ending there, not transferring there.

What is your EXACT route, and on which tickets? (The procedures can be different if you are on one ticket or separate tickets for a route).

Big airports usually provide big websites which can help the passenger navigate. You can Google both Heathrow and Frankfurt to get information, maybe even maps, for moving between terminals and gates.

Sorry this wasn't more clear: by transferring in Frankfurt I meant to catch our bus taking us to the first tour city, not a connecting flight.

We have 2 hrs between landing and the bus departing which, assuming everything is on time, should be enough, but I know Frankfurt is huge.

If it is Trier, trains go there too, pretty easy from Frankfurt.