Under an EU Directive, as long as the local garage does the work precisely to Kia standards, using precisely the correct fluids and parts, and invoices the work with a precise description of what was done and which parts were used, you will not invalidate the warranty. However, you will miss any Kia updates that would have been carried out as part of the service and, if I had a seven-year warranty that there was any chance of losing, I'd pay the extra to have the car serviced by a Kia dealer.