The most important thing about allied actions towards Spain during the civil war was the context in which they acted. With the rise of Hitler as an aggressive rising power in Europe, Britain was firm in its policy of appeasement, hoping to avoid a repeat of the first world war. The rise of a fascist Spain with Franco, therefore, was of interest to Hitler in his desire for similar fascist powers in Europe (shown by their strong interest in the war) and Britain did not wish to cause confrontation over it, pushing for official non-intervention.

Furthermore, the Republican socialist Popular Front which won the elections of 1936 was largely hostile towards foreign business interests in Spain, which had largely been exploiting the cheap labour Spain offered in its largely primitive industrial and agrarian policy, as well as the mines in the north. Thus, British economic interests, more subtly than their diplomatic interests, influenced their apparent friendliness towards Franco and against the Republic. This is also apparent in the upper class support for Franco, contrasted with the popular opinion in favour of a more worker friendly Republic.

France's position differed quite a bit, yet was ultimately influenced by British policy in Spain. France, like the Second Spanish Republic of 1936, had an elected Popular Front government under Léon Blum. Initially in the uprising in Spain, direct intervention in favour of the Republic was considered and debated intensely in the government, owing to French fears of a third fascist power on their borders and the collapse of another Popular Front government. However, France could not risk its relations with Britain, whom it needed to counterbalance the rise of fascism on its own borders. As such, French policy towards the Spanish civil war was one of official non-intervention, although a blind eye was often turned to arms and volunteers crossing the border to aid the Republic.

Also, the volunteer brigades on the side of the Republic, known as the International Brigades or Brigadas Internacionales, were not at all condoned by the countries that they came from, with the exception of the Soviet Union. The international brigadiers often ran the risk of heavy punishment back home for their service, and many had already been effectively exiled from their home countries for their opposition to fascist governments at home.

Sources:

Seidman, Michael. Workers Against Work

Preston, Paul. The Spanish Civil War: Reaction, Revolution and Revenge

Beevor, Antony. The Battle for Spain