Hello, Nun. First, please check your VAT settings by going to Account and Settings > Taxes. Make sure that VAT is still enabled. If it has been accidentally turned off, the tax field will not appear on invoices and bills. If it was enabled, but you still don’t see these options on your bills and invoices, you can also check the small gear icon located just above the Category column on your bill or invoice. Ensure that the box for VAT is ticked so it displays on your screen.
You can check this picture below for reference:

Regarding the change from pounds to dollars, check the specific profile of the vendor or customer involved. If your customer or vendor is set to use USD, the system will override your default currency for those specific transactions.
If both of those settings are configured correctly but the issue persists, try signing in using a different browser or an incognito window. This can help determine if a browser cache error is causing outdated information to display. Once you're logged in, check if the correct currency and tax options have returned to normal.
Let me know how it goes for you.