Foreign investment appraisal - Example 4 James Plc - Discounted cash flow

In this example - when calculating the tax payable - I noticed you split it into 2 parts - tax payable in oblivia (20%) and extra UK tax (5% after converting from Euros to £s). This is due to the double taxation treaty. While I understand this principle - could we not cut out a step and simple tax from the outset 25% which would leave the same net figure after converting to £s? Would we lose marks by doing this? Thanks