Currently if you make B2C (Business to Consumer) supplies of services from the UK to the EU, you have to charge UK VAT, because the place of supply is the UK.
But if you make B2C supplies of services from UK to outside EU, no VAT is charged as in this case the place of supply is the place of the customer, so it is outside the scope of UK VAT.
The same applies the other way round: when an EU supplier makes supplies of services to a UK customer, EU supplier charges EU VAT.
Post Brexit transition period
B2C supplies of services from UK to anywhere else in world will be outside the scope of UK VAT.
Similarly, no EU VAT will be charged on B2C supplies of services from an EU supplier to a UK customer.
Charities that have no business activities are “C” so if they receive services from the EU, that is B2C. So from 01/01/21 they will not be charged EU VAT. Neither will ordinary consumers. I suspect there will be a rush to set up professional services firms in Ireland to supply GB and NI charities and consumers.
For details on which services these relate to, See VAT Notice 741a Section 12.