It shouldn't be difficult to install a genuine mugen airbox. Is yours genuine? Do you have any modifications on your car that might causing your issues?
20% VAT and 10% Custom duty is payable on the total paid, including shipping costs.
I think you will pay close to £300, as royal mail/parcel force also charge a handling fee on top
It's my understanding (being Australian as well) is if you own the car overseas for more than 12 months, you can import the car using the personal import laws and avoid any tax etc.
Is this it?
https://www.gumtree.com/p/cars-vans-motorbikes/honda-civic-16-b16b-vtec-ek9-import/1348920919?utm_source=savedsearches&utm_medium=email&utm_campaign=GT.TRIGGER.SA&utm_content=none&utm_source=savedsearches&utm_medium=email&utm_campaign=GT.TRIGGER.SA&utm_content=none