I'm surprised, the homehub 3 has fantastic wireless performance! I used to be able to get full VDSL internet speeds from the attic of a 3 story house. I'm not sure what they're like for WDS (Wireless Distribution System), but that may be an option. It works by taking a wireless signal and repeating it, rather than taking a wired signal. They can be a bit hit and miss on compatibility though.
See if you know anyone local that could borrow you some powerline adaptors to test might be your best bet. Or buy them from somewhere you would be able to easily return them, if they don't work!
If you're with BT, it may even be worth considering BT Vision! When I got it, I received a pair of powerline adaptors with the Vision box for free. The newer versions of adaptors they give out now are awesome too! It'd work out £48 for the 12 month contract.