Applying to theoretical physics Masters in Europe while not having a physics undergraduate degree
This is a tough case. In many European universities, the expectation for master's study in a given discipline is a bachelor's degree in that discipline. Whether or not it is possible to petition for admission with another degree is up to the regulations of the particular university. However, if you do not have formal coursework available demonstrating that you have the requisite background, it is unlikely that you will be able to get approval. 

Your best bet might be to look for programs that are interdisciplinary in nature, as they tend not to have such stringent requirements.