It looks like (from the comments in the thread you linked) this map is based on a list of Roman cities that was taken from wikipedia: https://en.wikipedia.org/wiki/List_of_cities_founded_by_the_Romans

As often happens with wikipedia's crowd-sourced data, this list is very incomplete and as you can tell is very heavily slanted toward cities in Britain (probably because the wikipedia users who compiled the list were more familiar with sites in English-speaking areas versus other parts of the non-English speaking Mediterranean world).

If you want to see a more accurate (and interactive) map of the Roman world, there are a few cool online projects you should check out.

• Pelagios Commons has published a very detailed map of the Roman world that combines several large academic databases of Roman sites that have been painstakingly assembled by historians and archaeologists--a much more comprehensive list than the wikipedia page. You can view the map here: http://pelagios.org/maps/greco-roman/

• DARMC. The Pelagios map uses data from DARMC, the Digital Atlas of Roman and Medieval Civilizations, hosted by Harvard. DARMC has their own interactive map, and you can use this to filter specific kinds of data that appear on the map (only Roman forts, for example, or cities + roads, etc). It's a lot of fun to play around with: https://darmc.harvard.edu/maps

• ORBIS. Finally, Stanford hosts a map of the Roman world called ORBIS that lets you calculate travels times and routes between locations, much like you do with google maps today. It's also a lot of fun, and helps show how different travels times and routes were in the past than they are today. You can find it (and play with it) here: http://orbis.stanford.edu/

All these maps are more reliable and much more detailed than the one shared in that reddit thread.

Roman Britain was heavily populated during the Roman period, but it was a comparative backwater next to the Mediterranean. British cities were small, and most of the population were poor farmers who lived in the surrounding countryside and struggled to grow enough wheat to keep up with their taxes. You can still find hundreds of ruined Roman sites (mostly villas--big plantation houses where Roman elites lived) across Britain, but it's nothing like the much more developed heartlands of the empire in Italy, Africa, and of course the Near East.

Are you sure that those dots represent "Roman settlements" (and what the hell is the date?)? If they do the map is utterly mistaken. Ramsey MacMullen's Romanization in the Time of Augustus lists some 38 new colonies in Africa alone by the death of Augustus. This is Roman Africa, mind you, the actual province, so it does not include later settlements in Mauretania or in what is now Libya. The map here, plotting...whatever it is exactly it's plotting, shows nothing in the province at all. He charts a further 77 in Spain at the time of Augustus, 35 of them in Baetica alone--by contrast, the map there shows only three "settlements" (colonies? Military camps? What are these "settlements" exactly?) in Baetica. The situation is made worse in the thread from which this is taken by a disregard for clear definitions of what exactly is being tracked here and a willingness to say things about habits of Roman settlement that are not true (the Romans certainly did not regularly raze cities to the ground and rebuild over them...)