Because of the risk of malaria in Thailand we are now considering going to Morocco. I have read some interesting journeys by car in this country. When I searched on Google image for "traffic signs morocco". There appear to be quite some Arabic traffic signs. This make me wonder if it is a safe to rent a car in Morocco while I don't understand Arabic.
There is no problem driving in Morocco if you cannot understand Arabic. I think the impression you get from your Google query is not quite correct. The road signs are bilingual Arabic/French, sometimes even with pictograms:
In cities the signage can sometimes be a bit meager. Combined with the Moroccan style of drive, this makes crossing a city a bit more stressful. For overland travel, the signage is fine. Nevertheless it is a good idea to have a road map.
You certainly are not the first English-speaker who will rent a car in Morocco. In addition, all panels and official papers are in Arabic and French (French must be more readable for a English-speaker than Arabic, I suppose).
The only danger I see for an English-speaker coming from UK, is that in Morocco we drive on the right side, so if you're used to driving on the left side please be careful.
If you are concerned about communication with local people, English is not completely absent in Morocco, it is officially considered as the second foreign language in the country after French. All students must have English courses starting from the first year of high school to the 3rd year (unless if they choose English literature in University).
If you go to a touristic city or place like Marrakech, Agadir, Fes, etc, English is a must for guides and workers, so I don't think you will have problems at least in those places.