Are trans Atlantic flights which pass over Iceland normally controlled by "Reykjavik Center"? I would imagine these flights would normally be assigned a North Atlantic Track (NATs) and Icelandic control only goes up to a certain level?
Now that part of Icelandic airspace is closed, are the NATs effected? are certain NATs not available? Do the oceanic centers create ad hoc alternative routes, or does "Reykjavik Center" step in and coördinate with the respective oceanic centers?
BA49 LHR-SEA currently seems to be taking a southern route past Iceland and then a different route north over Greenland.
NAT tracks predominately cater for traffic between Europe and the eastern seaboard, and as such usually route to the south of Iceland, and as such do not usually speak to Rekyavik, but to Gander and then Shanwick.
It is worthwhile looking at a Great Circle website to demonstrate the shortest route between two airports - the shortest time route will of course be affected by the upper level winds of the day.
Typically, when I operated trans-Atlantic from the UK to North America, I would not use NAT tracks when flying to the US west coast/Canada and sometimes/usually not en-route to Miami / further south.
Does this help?
For information, Rekyjavik FIR is north of 61N from 0W to 30W, and from N61W30 in a direct line to N63.5W39