mangento 2.3 Uk Tax subtotal incorrect

Question

I have a store set up on the Uk with the following 3 tax classes 20%, 5%,0%

I have set the tax rule for the uk

I have set the Shipping settings for the Origin as UK

On the shopping cart I have set the prices so that they show as Ex VAT on the catalogue process and also set the same for the shopping cart. The Shopping cart total I want to show the included tax price

On the shopping cart I select the UK. On the tax after the subtotal it has calculated the wrong amount as the correct tax amount should be (£100+20%= £120) I used this tax calculator to check what the correct tax should be as this calculator shows http://www.vatcalculator.co.uk/