Hi Winther01,
If you are providing a service of working on goods then this supply will come under the General Rule when determining the place of supply.
If your customer is in the UK then the place of belonging, of the customer, determines the place of the supply.
The place of supply would, in your case, be the UK and hence UK VAT would normally be charged.
Please see the guidance at Section 15. Zero rating and specifically section 9.7:
Place of supply of services (VAT Notice 741A)
However, if you are working on goods which you then send overseas you can Zero rate the supply as per the guidance at Section 15 and specifically section 15.9:
Place of supply of services (VAT Notice 741A)
If the goods are exported then you will need this export evidence in your records to support the zero rating of these supplies
Thank you