This is based on research into buying in Holland, so please excuse any errors.
If the model is sold into the UK it shouldn't be a huge problem. I'd suggest using a shipping agent, but customs are simply going to take a look, value and charge you any applicable duty. The agent is good as they might fight your corner if the VAT man decides your bike is worth £10,000. With new stuff you know what you'll pay as you have a dealer receipt, for second hand guess or print some pages off e-bay. If you can get UK insurance you should even be able to ride it home and to the MOT on the Australian plates.
Once the bike is here you've got to register it. If you've got the Australian papers that show the model and age, you take it for an MOT (inspection of mechanical condition when over 3 years old in case you didn't know) then go down the local tax office and apply for a registration. With age related papers they'll issue a V5 and you can get a plate made with the correct age letter. With V5, insurance and MOT you can pay your road tax and your there.
Hassle I think will come in three places:
If you can't prove the age they'll issue a Q plate. This is no great shakes except your insurance costs go up as you'll get lumped in with all the people who've welded 3 Trabants and a Lada together to make a 4x4 limo.
If the model isn't on their list they'll want it to go through SVA, single vehicle approval. This is a glorified MOT where they'll look for tyres without EU marks, fuel caps that stick out too far, lights that dip incorrectly etc. From this you should only get a bill for new tyres etc. on an SRX but on some bikes you'd be stuck (for example if no one makes approved tyres). Enfields brought in from India without going through the official importer for example need SVA because the home market model is known to need mods.
The MOT might pick out some unexpected bits too. Going the other direction for example I believe UK bikes have to have wear markers fitted to drum brakes to pass the Australian MOT. I can't think how a well set up bike would fail, but you might need to do some work.
This is a lot of hassle and I'd agree with the suggestion of buying here and sorting a UK bike, but good luck if you go for it,
Andy