In the BBC series Father Brown, which takes place in the 1950s, the eponymous catholic priest is shown on multiple occasions wearing military medals he had recived for his service in WW1 (before he became a priest). He either pinned them to his cassock picture or his stole. Would a Catholic priest have been allowed to do this? And if so, how common was it for priests to wear medals with liturgical/clerical clothing?
No...... Catholic liturgical garb is pretty strict. The medals would count as "profane", and would not be suitable for use in the Liturgy.
Although, as an amusing aside, the practice is not without precedent. The founder of one American seminary was a prussian veteran who also wore his medals on his cassock
