Yes! Classical philosophers and writers such as Diodorus Siculus, Sotion of Alexandria and Strabo considered the Gallic druids to have been natural and moral philosophers. Unfortunately we do not know much about their actual philosophy besides scraps of information passed on by Greek and Roman writers.

The druids themselves appear to have been important figures in early Gallic, British and Irish societies although it's not certain if these were parallel institutions or some kind of common priesthood which Caesar's work seems to suggest, but some scholars have argued that his portrayal of the druids was largely propagandistic and invented. According to Caesar, the druids were recruited from young Gallic noblemen who learned their religious and philosophical doctrines entirely orally, the reason why we know so little about their beliefs. He claimed that the druids acted as important priestly figures in carrying out sacrifices, worshiping various divinities and in "the interpretation of ritual questions" - which sounds a lot like theology or philosophy.

So what do we actually know about the druids' philosophy? The most consistent fact throughout the corpus of Classical descriptions of the druids is their belief in the reincarnation of the soul. According to Caesar: "They wish to inculcate this as one of their leading tenets, that souls do not become extinct, but pass after death from one body to another, and they think that men by this tenet are in a great degree excited to valor, the fear of death being disregarded." This belief in reincarnation seems to be confirmed by a Celtic source - the early Irish narrative text, In Cath Catharda (which is itself about Julius Caesar's Civil War) describes the "druids" as one of the many nations who join Caesar's host before his march on Rome. The text makes a brief description of druidic belief in the Irish context:

Came there the people of the Druids— that people whom science and soothsaying used to serve to find out (the future) from the courses of stars and constellations; and this is what they would say through the diabolical sciences, that the souls of those that died in this (northern) temperate zone were taken southward through the torrid zone, and placed in other bodies in the southern temperate zone. Druids the names of those folks, and Druis the name of their city. Hence is said druí 'druid' and druidecht 'druidism' in Scotic (Irish).

It's possible that this description of the druids was cribbed by medieval Irish writers from Classical texts, however the detailed description of the druidic belief in reincarnation seems to be unique, perhaps indicating continued knowledge of druidic doctrine in the medieval period (and, as we will see, the druids themselves continued to exist in Ireland in the early medieval period). The druids' knowledge of celestial bodies and their movements combined with a philosophy of the natural world is attested by Caesar as well: "They likewise discuss and impart to the youth many things respecting the stars and their motion, respecting the extent of the world and of our earth, respecting the nature of things, respecting the power and the majesty of the immortal gods." Astronomy, then, seems to have been the foundation of druidic philosophy and religious belief.

In both Gaul and Ireland (sorry, don't know much about Britain), the druids survived after the Roman conquest/later outlawing of druidism and the conversion to Christianity respectively albeit in different roles. In both situations, the druids appear to have been relegated to seers, healers or sorcerers who lived on the margins of society. In Ireland they seem to have been a nuisance to the early Irish church: at least one ecclesiastic text from the 6th or 7th century had to remind the Irish that performing oaths in front of druids was forbidden. Their marginal and subversive status was reflected in Irish law as well; Irish customary law from the Christian era maintained that druids had the same legal standing as brigands.

Sources:

Julius Caesar, The Gallic Wars Book 6, trans. W. A. McDevitte and W. S. Bohn.

"In Cath Catharda: The Civil War of the Romans" translated by Whitley Stokes.

Jean-Louis Brunaux. Les Druides. Des philosophes chez les barbares. Éditions du Seuil, "Points Histoire", 2006.

Fergus Kelly. A Guide to Early Irish Law. Dublin: Dublin Institute for Advanced Studies, 1988.