In Iceland you can hire an English-speaking guide notably through Evaneos-Travel.
You can travel to Iceland independently, and organise it quite easily with an Evaneos agent. The country is considered one of the safest in the world, so there are no major concerns for those who want to travel alone. However, if the idea of exploring the vast tracts of wilderness, especially in the heart of the country, does concern you, or if you simply want to learn more about the country during your stay, it may be wise to hire an English-speaking guide.
It is wise to travel with a guide if you really want to learn more about the country, or if you have interests that could benefit from a person who really knows their stuff. This is especially the case for ornithologists or geology enthusiasts. If you plan to visit Iceland in winter and you are not too confident on icy or snowy roads, again a guide or driver is very useful option for exploring outside of Reykjavik.
Finally, in Iceland, a guide may prove especially useful for hiking because of sometimes poorly marked trails, sudden changes in climate that may be alarming to those who are unfamiliar with them, and of course the comfort of not being afraid of getting lost or walking in the wrong direction.