CAP 747 Appendix 1 GR NO 10 states that for a UK C of A aircraft a uk certification is required or specific permission for an overseas company to certify has to be obtained from the UK CAA.
So it doesn't matter what approval the overseas company hold they cannot certify without permission.
The above document is a UK generic requirement which has similar weight to an AD. So ignoring it might make you C of A invalid.
A UK approved company or licensed engineer can certify on behalf of the overseas company.
www.caa.co.uk/docs/33/CAP747.PDF
go to appendix 1 GR No 10