The restaurant at the Blue Lagoon was very helpful. In general, the serving staff spoke very good Enlish and were very helpful. It was reasonably easy to buy food in the supermarkets.
It looks expensive but the main courses are quite large and it is very acceptable to just order tap water (which is pure spring water and delicious). Also the puddings are mainly not GF or are skyr (like a yogurt) so we'd go back to our accommodation to eat our own shop bought skyr for pudding and drink alcohol (sometimes). So just ordering a main course each and tap water is not all that expensive really.