Please direct any tax or customs-related questions to Shopify’s support team.
Shopify support is briefed on tax-related queries, and compile these types of concerns internally to make sure the platform is doing what it needs to (and its message is clear). This includes the idea of showing extra ‘tax not included’ text by prices, which may be a setup issue requiring no theme alterations on your part.
Contact Shopify Support: Click This Link
Below are common Tax related topics on Shopify that may apply to your question.
Include or exclude tax based on your customer’s country: Click This Link
UK Taxes & Brexit: Click This Link
Tax Overrides & Exemptions: Click This Link