GOV.UK
General provisions: What does this mean in relation to supplies received by a UK member of a CSG based in another Member State?
If the UK member is receiving the supply in relation to a business the place of supply is the UK and as the cost sharing exemption is implemented in the UK, subject to all the conditions being met, the supply will be exempt with no need to apply the reverse charge.
If the UK member is not in business for VAT purposes the place of supply will be the Member State where the CSG is established.