You should talk to an accountant about this - who or what document has told you you owe 20% on your non-EU imports at VAT return time? Normally with imports from outside the EU you don’t have to do anything special on your VAT return - the import VAT, if there is any, is charged up front and for postal imports it is collected by the carrier before they release the parcel. It’s only EU acquisitions where you have to declare the 20% on your VAT return.
(This may all change post-Brexit of course, but this is how I understand the current situation)