This is not a complex routing by any stretch of the imagination

Just call up and feed the agent the exact flight numbers you want to be on, including dates and they should be able to find the price. Certainly this is simple enough to not need one of the better agents to complete - even one of the useless ones should manage! If it helps, give the fare codes for each flight to help them along.

You will likely be quoted in EUR, but you can ask to switch to another currency (this is not something I have ever personally done, but someone will advise better).

Best to call when the UK (NCL) office is open.

Good luck