The H&H shells will likely work in a Flues . . . but you might not like them in comparison to RST 2 1/2". Most British shells we can get in this country are field loads. As such, they can be a bit on the "hot" side if you're shooting a lot--at targets, for example. The RST shells are both true 2 1/2" and usually at a more moderate velocity than the British shells we can get here.
The typical problem with 67-67.5MM shells in a 2 1/2" gun comes with old (pre-1900) doubles that don't really have a forcing cone, but rather just a "step" from the chamber into the barrel. In those cases, you'll get blown ends and likely sharp recoil from the longer hulls.
