If you are VAT registered in the UK, any supplier outside UK should not be charging you UK VAT. They can only charge you UK VAT if they are VAT registered in the UK as well.
You can check with your vendor, and if they are UK VAT registered, you can set the VAT treatment of the Bill to United Kingdom. You will then be able to enter 20% VAT for the bill.
To set the VAT treatment of the bill:
On the other hand, if your vendor is not registered for VAT in the UK and, if you are selling a service, you should be reverse charging this bill. If it is a bill/expense for goods that you buy, Acquisition VAT is applicable for a EU vendor and Import VAT is applicable for a Non-EU vendor.
In that case, you can simply select 20% VAT and Zoho Books will make the reverse charge entries for you. Reverse Charge will only be applied on ‘Service’ purchases and not on ‘Goods’ purchases. If it is a ‘Goods’ purchase, no VAT should be applied and it should be recorded as Import VAT.
You will be able to see the text Reverse Charge under the VAT rate. You need not change the VAT treatment of the vendor in such a case.