Last weekend, whilst driving back to England I got flashed by a speed camera. I was doing 120 in what I believed to be a 120 autobahn zone, even my satnav also stated it was a 120 zone. However, after being flashed I then saw the 100 sign. I have obviously broken the speed limit and as I want to return to Switzerland, I do not want a fine hanging over me. So where should I go or who should I contact to pay it? My car is GB plated and registered vehicle. I don't want to simply wait and see and later find I have court costs, etc added as well.

i think you should find out in which Canton you were flashed and then contact the cantonal police.
alternatively, you can wait for the find to come to your home address.

Well did you actually yet receive notice of a fine, through the mail? If not, just wait for it to come. Until you get this, there is no need to do anything at all.

Ensure wherever your car is registered you are receiving the post for that address. If they do send, it may take sometime.

Do you know exactly what speed you were doing if you thought it was 120?

If you were doing less than 120, they may not send, it very much depends on where. Something like a 50/50 chance.

If you were doing 140, more likely they would send.

What about a ticket sent to the States? A friend of mine got a hefty fine in a rental car with less than one second over a red light (around chf 250 I think), and it made its way to him in the States. The question was what would happen if he didn't pay it (and never came back to CH). I was thinking the rental car company might get charged and charge it to his card?

From the experiences of other EF members that's exactly what will happen.... plus the rental company can add administration fees on top of the original fine.

That is not a hefty fine for this country

The UK is currently opted-out of the agreement to share driver registration data. It means you will probably never get a ticket.

In my first year in CH I had a UK registered car which was flashed routinely on the highways, but I never got a ticket.

Iron Pig, (Slammer's motorbike) got her first Maltese parking ticket today for 23,50€'s, as she still has her Swiss plates I wonder how that is gonna play out

As plod has the renters data the rental company will have already added a nice administration fee to the bill.

Fixed your typo