You should be able to get the minimum legally required Euro "green card" insurance for an OZ-registered bike to ride, starting/ valid in the UK, across Western Europe from:
Green Card
or
How does the European Green Card Insurance work?
or
TourInsure - Online Application forms (already mentioned above).
You should be able to apply to any of the above by email/ online/ phone while you're still in the UK, and they will arrange/ send a valid policy to you in the UK before you leave.
Give yourself at least a week to arrange this paperwork with any of the above before you set out from the UK. Actually, if you buy it "now," you'll be covered in the UK, for however long you plan to remain there, until you leave (but you'll be paying on a monthly basis, or min 15 days from Mototouring.com)

It's confusing because, when you approach UK vehicle insurance underwriter/ agents, they don't write for non-UK registered bikes, and the UK auto clubs (RAC, AA) also don't write separate Euro green card policies for non-UK vehicles. Brits are peculiar that way (but then so are Americans)

After "Western Europe," it's "border by border."
Depending on your routing--if you ride across the Balkans (Croatia, Bos-Herz, etc) you're "supposed" to get country-specific cover starting at the Slovenia->Croatia border, but it's not always available depending on border crossing. Just ride thru with that extra risk, if necessary. Serbia may not allow your bike to enter without their extra 60Euro min 30 day Serbia-only cover; and it's available only at their major border crossings.
But if you ride Romania-Bulgaria, you're covered to the Turkey border; or if you ferry Italy-Greece, you're also covered to the Turkey border (where border insurance is again available & required).

good luck
p.s. above assumes you're planning to ride eastward thru Europe; if you're instead shipping to North America, you can get NoAm insurance for your OZ-registered bike from TourInsure.de, or MotorcycleExpress.com.