Originally Posted by pkoo

If you're transiting at a Schengen country, you won't receive a stamp, as all inbound/outbound gates are located in a single area beyond immigration control. FRA is a bit odd in that terminals are separated by carriers & partners instead of destination, which means you might have to walk a bit but as long as you don't exit the non-Schengen area, you won't have to go through customs though you probably will have to go through security screening if your outbound gate is in another "pier."

In this case, you will receive a passport stamp on these occasions in this particular order according to your trip:
1. entry into UK
2. exit UK
3. entry into Schengen
4. exit Schengen