I just thought I’d update this answer with a final resolution.
It turns out Shopify can - on request - disable the automatic price adjustments based on geolocation feature on a per-store basis. It is not a setting store owners can see or manage, only Shopify can turn it off. It appears to default to “on”, since I had to explicitly ask for it to be disabled on all of the stores I run.
Once turned off - prices are shown the same for everyone that visits the website until they confirm where they are shipping the products, which is the logical configuration since it is shipping destination that dictates whether VAT is added or not (at least in the UK, shipping internationally).
Once Shopify turned off this setting it obviated my fabricated Google pricing code shown above, so please disregard it if you’re trying to achieve the same goal as I was. Instead, just contact Shopify and ask for this feature to be switched off.