Well, well, well, this is kind of my speciality, for once! Quick disclaimer, though: I'm interested in urbanism and the development of cities and their local institutions throughout the middle ages. My scope, though, is mainly concerned to the territories of the former Crown of Aragon and, as such, most sources I'll cite are in catalan or spanish. If that's not acceptable for the sub, feel free to erase my comment. Let's begin, then!

As you are probably aware, street names were always informal and were subject to much variation across time. As such, one must keep in mind that up until the 19th century, the public space is created before its name, even if it was a planned urban development.

In a typical medievall walled city, open spaces need to have a definite purpose to be maintained through time as population soared. The purpose they could have ranged between being vegetable back gardens needed to feed the population in case of a siege or quarantine, communication between the exterior and private residences or, specially relevant to your question, economic activity.

In the case of the cities, towns and villages in the Crown of Aragon, there exists a dichotomy between those with a "Carrer Major" (High/Main street in catalan) and a "Plaça Major". There's no place, to my knowledge, that had both at the same time. Some have even neither of these names, though which street or square is the "Major" one is clearly distinguishable. That is, the function of a Carrer Major or Plaça Major is clear even if they don't bear that name.

Let's examine the city of Tarragona, my hometown, and it's relationship with open spaces as a small medieval city. Tarragona's old quarter has about 10 squares of various sizes nowadays. Most of them are quite modern and date fron the 19th century onwards. There's only 5 that have a medieval origin. Of those:

• One was a tiny square in the Jewish Quarter, where christians didn't usually go
• One was the Royal Castle's Courtyard
• One was created in front of our Cathedral in the 1330s by tearing down some houses because one of our Archbishops wanted the church to be seen from afar.
• One was a large space in the middle of a 14th century expansion of the city which served the purpose of holding fairs.
• And the last one is the oldest one. A tiny square at the end of our Carrer Major, enlarged in the 16th century to its current shape.

The purpose to that last square was to communicate the streets of the city where market was held (Carrer Major and Carrer Merceria form an L with said square at their intersection). In our case, then, the city was formed with a (two, in fact) street, and not a square, at its main axis. This is actually the case for most older cities in the area, one of which is Barcelona. Barcelona's old quarter is centered around a street as well (though it's not called Carrer Major, there and there's a 19th century square in the middle now).

On the other hand, we can observe that places in the Crown of Aragon which have a Plaça Major from where the city expands in all directions were usually minor walled settlements that held their markets outside the settlement's walls in the medieval era. Once the settlement grew in, usually in the early Modern era, it maintaned a functional open space reserved for commerce, which now lies at the center of the modern town/city. This process can be usually seen quite clearly in medium to small towns, where one can see that the expansion around an open space outside of the walls started but got stumpted for whatever reason and an open space has remained clearly not in the center of the town. This is visible in places like Prades, and also in places like Vic and even Tarragona, where a big square is clearly off center when only taking into consideration the old medieval quarter.

In some cases such as Reus the use of the Main Street (Carrer Major) as a comercial way persists through time alongside new open spaces, and these open spaces never adopt the moniker "Major". The big square at Reus' center translates as "Market Square" for instance.

So, as you can see, it's not as clear cut as Continental western Europe going for squares while Britain went for streets. In fact, regardless of what I told you, you could come to my hometown and think that one of our "modern" squares was the center of the medieval and Early modern town when it was not, so it can also be a matter of foreign perception.

There's also, I believe, an important climatic factor at play which I'm not knowledgeable enough to discuss. Maybe Britain's and the CoA's weather was a factor for streets being the main axis in cities and towns. I haven't read anything about that, but it may very well be, as I'm afraid I'm not that knowledgeable beyond the scope of the Crown of Aragon.

I hope I have made myself understandable. English is not my native language, after all!

Sources:

Rovira i Miró, Aleix. Les institucions municipals del Camp de Tarragona en l'època medieval: el cas de Tarragona i Reus. Dipòsit de la Universitat de Barcelona, 2018.

Sabaté i Curull, Flocel. El territori de la Catalunya medieval. Percepció de l'espai i divisió territorial al llarg de l'edat mitjana. Fundació Salvador Vives i Casajuana. Barcelona, 1997.

Pirenne, Henri. Las ciudades de la edad media. Alianza editorial. Madrid, 2015.

Dantí, Jaume (coord); Serra, Eva; Gual, Valentí; Alcoberro, Agustí & Font, Jaume. Ciutats, viles i pobles a la xarxa urbana de la Catalunya moderna. Rafael Dalmau, Editor. Barcelona, 2005.

Juncosa i Bonet, Eduard. Estructura y dinamicas de poder en el señorío de Tarragona. Creación y evolución de un dominio compartido (ca 1118-1462). Consejo superior de Investigaciones Científicas. Barcelona, 2015.

Dantí, Jaume (coord); Pojada, Patrici; Gómez Zorraquino, José Ignacio; Lobato, Isabel; Gual, Valentí; Jorba, Xavier; Torras i Ribé, Josep M. & Alcoberro, Agustí. Les xarxes urbanes a la Catalunya dels segles XVI i XVII. Rafael Dalmau, Editor. Barcelona, 2011.

PS: Sorry, I don't know to put this in a nice format from my cellphone.

Edit: typos