Hi I am in UK too and I am having this problem..
My web was fine all this time for last few months. in UK for last year we had VAT 15% now change back to 17.5%. so I thought it would be so simple of going in admin panel, virtuemart and than change tax rate.I have manage to do so how ever after the change the cart behaving weird.
I have got 4 step simple cart. the 4th step (preview order page is absolutely fine) but on page (steps of checkout cart) page 1,2,3 the sub total is wrong.
unit prices is right which is £10.00 VAT (tax) is right which £1.75 BUT SUBTOTAL is wrong £10.22 and again Subtotal: is wrong £10.22
Total VAT: £1.75 OK
Total: £11.97 The Calculation is ok
this happens to checkout steps 1, 2, and 3 the 4th step is fine....
also after changing tax the checkout steps and last step the tax showing ok but when I get confirmation email, the tax on that is still showing old 15% and not 17.5%
Thanks in advance