This is because of VAT issues caused by the Brexit. We are obliged to separately submit a tax return for the UK and have to be able to accept UK VAT numbers from customers. At this point in time the HMRC API is not working properly to check VAT numbers if they are indeed valid.
Once everything is working properly, you can select UK as country again.
If the issue is solved, it will take quite some time as in months.
Currently there are a lot of requirements from HMRC. We are still looking into this how we can efficiently solve this and what it will mean for us in the future.