Why has my monthly invoice from QuickBooks double from £8.40 to £16.80?
Welcome to the Community page,
Having looked into your account, the reason in which your subscription has doubled is that you were on a discount of 50% which expired on 19 March 2020. So the subscription is now gone to the full price which is the 16.80 you are currently paying.
Your discounted period is expired and you have to pay the regular price now. If you wish to keep the lower price, you will need another effort. Utilize the 3rd party tool to migrate your data between QBO accounts.