If I order some goods over the internet from a UK company and I am
based in the UK then who has to pay return postage if the goods are
unsuitable?

Does the Distance Selling Directive (or perhaps even the Sale of
Goods Act) specify who has to pay the postage for returns made under
such circumstances?
This is down to the terms and conditions on the retailers website,,,

No it isn't. The T*C cannot override the SOGA or DSA !!