It is very much dependent on the type of volunteer. Orwell originally went to Spain to write articles and was swept up in the revolutionary air in Barcelona at the time. The Spanish Civil War was very much a partisan wars for those outside with little knowledge of the context of the previous century in Spain. This happened with a few journalists, which tended to come from middle class backgrounds.

Quite a high number of volunteers who went to Spain with the intention to fight against 'fascism' tended to be workers from areas such as Manchester, Liverpool and across Wales. If you are in England and are near Manchester, I would highly recommend going to the Workers Museum in Manchester. They have a fair bit about British voulnteers in the Spain.

Sources: Homage to Catalonia, Orwell Eisenwein, Seeing The Spanish Civil war through foreign eyes.

If you are interested in more on the Spanish Civil war Paul Preston has written some good books on it.