The TAX is designed for QuickBooks Online US while the VAT is available in the UK version. Thus, we're unable to replace the word on your sales form.
Please see attached screenshots below for your guidance.
You may need to create a new QBO UK account since you're also unable to change the region on your current subscription. Please go to this link and choose your country: https://quickbooks.intuit.com/choose-country/, then sign up from there.
I suggest getting back in touch and discuss this with your account for additional guidance.
Keep me posted if you have follow-up questions. I'm here anytime you need help.