According to art 194 of the VAT Directive, Member States may implement an optional reverse charge on supplies made by non-established businesses. UK has only introduced this reverse charge on supplies of services located in the UK.
Where a non-established supplier provides services located in the UK to a UK VAT registered business, domestic reverse charge applies. It is not relevant if the supplier is registered or not.
Not established in UK
(irrelevant if the supplier is registered or not for VAT)
VAT registered in the UK
Supplies of services located in UK
More information about the scope and VAT treatment of this reverse charge can be found in the online materials published by HMRC.