The Roku 4 is a great box, but it has a couple of hurdles for Smart DNS users like yourself.
1) You need to put your DNS servers in your router as there is no way to do this on the device. (not all routers can do this).
2) You will need to block Google's own DNS servers via static routes (even fewer routers can do this).
3) You can't add apps from other regions. Which means if you ever wanted to also add US apps, you can't.
On the other hand, you will get all the main UK catch-up services and more:
BBC iPlayer (including live streams)
BBC Sport
ITV Hub (including live streams)
All 4
Demand 5
Plus pay services like
Now TV
Netflix
Amazon Prime UK
The Amazon Fire TV on the other hand does not have any of the above problems. You can easily add your Smart DNS directly to the device and you don't need any static routes. You can also add American apps to the UK ones in order to get even more content (but this requires a little trick).
On the negative side, there is something significant missing: You get:
BBC iPlayer (including live streams)
BBC Sport
ITV Hub (no live streams yet)
Demand 5
UKTV Play (with Dave, Really, Yesterday and Drama)
TV Player (live streams of above channels plus Channel 4 for free, plus pay channels like British EuroSport).
Along with:
Netflix
Amazon Prime.
As you can see, All 4 is missing! No one knows if it will arrive, but the general consensus is that it must at some stage - we just don't know when.
If all you want is the main 4 networks, then instead of the Roku 4, why not a Now TV Box? This is a "Roku Powered" Roku 3 that Now TV sells for only £15. It has everything the Roku has mentioned above it the free section, and Sky never hound owners if they don't subscribe to Now TV. They have free delivery in the UK, so you will need to get a friend or relative to send it to you. After-all, you don't really need 4K right now as none of the FTA catch-up services use 4K, and even if they did, you would never be able to reach those heights from Australia.