New to ST, not set things up yet but about to, I’ll have a V2 hub, Aurora Lighting dimmers, door/window sensors and motion detectors, Arlo Pro 2 cameras.
The question is: Does there exist an external Zigbee or Z-wave mains wired siren (but preferably battery back up) that I can integrate? There seems to be conflicting information. Or could I use an Aurora smart relay in line to trigger a non smart external siren?
See the following thread. (The topic title is a clickable Link).
Using An Aurora relay would be the same as the smart plug solutions described in that thread.
Thanks JDR, I’d looked at that thread already although it’s 18 months old now so was hoping for some more up to date information. Plus there were links to US sites so I’d be unsure I’d certain things were compatible with a UK hub?
You can consider using Aeotec Siren it is Z-Wave with an EU plug with built in battery and Strobe light.
I am using the earlier model Aeotec Gen 5 siren but I believe that they have a newer Gen 6 now.
It works fine with my V2 Hub - detected and installed with no issues.
Thanks Chak, but this is an IP20 internal siren with European plug connection, not IP65+ for outdoors with hard wire waterproof connection.
The most recent post in that thread is only a month old, so it’s still current information. 
Occasionally US members do post into UK threads without realizing that it’s a UK thread, but there are several UK members posting in that thread as well.
If you have any questions about any specific devices mentioned in that thread, just ask the question in that thread and someone will answer.
You are right that the UK hub and the US hub operate on different Z wave frequencies and you do have to match the end devices exactly, but there are many Z wave manufacturers who do create a US model and a UK model, so it may be that the UK model would be available from a UK source. But just ask, and people will answer in that thread.