HPSOV L.
I would also go with GF if he has received written confirmation from the revenue office. Like you said, it gives you good grounds against tax evasion.
Regarding other countries as you mentioned: I have been in this 'expat' game 15 years now and many other countries, at least in the EU anyway interpret the double tax agreements the same way that GF has mentioned. While friends/colleagues in the UK have been topping up tax, guys on the continent have been 'signed off' by the respective revenue offices with tax paid abroad as sufficient proof under the relevant DTA. Interesting, maybe the UK is changing its ways now more in line with the rest of the EU. Who knows.
All the best