If the Sumvision doesn't do 5.1 passthrough then it's a licensing issue. Mele PCG03 and Sumvision are exactly the same hardware, manufactured by Mele.
The Sumvision link on our site goes to Amazon. We aren't going to stock it. I'm not sure whether anyone in the UK has Mele PCG03 stock now so you're going to be best off buying the Sumvision version.