Options with a Diploma in Theoretical Physics (A failed MSc)
To be blunt: not good. Top school or not top school, if you can't get an MSci, maybe you are just not cut for it. Yes, HEP theory is intellectually the most demanding field, but it is also currently in least demand and I've seen some very very good people leaving the field after pushing two postdocs. So really, if you insist on doing HEP theory and can't get an MSci, maybe it is good that you get cut of years of depression before it is too late. Another observation is that in HEP theory very few people push the field forward and the rest are completely irrelevant (i.e you write some stupid lagrangian, get eom, write not a very good paper and repeat; it is really not helping anyone. Very few other fields have this dichotomy)

Now, some people get into other fields and flourish. I know a person who had 1.5 postdocs in HEP theory, doing so so, some papers, some citations, but no real permanent job offers and he switched to biophysics, applied some easy peasy mean field QFT methods to neural networks and to them this was science fiction and now he is a star in that field. I also know another person, a stundent. who would do mind-boggling two loop integrals and wanted to do some data analysis and was so hopeless that it was embarrassing for all involved. So not all transplants are successful. 

My advice would therefore be: if coding/data/algo is something that excites you, think seriously of moving into data science while it is still hot. If deep questions excite you but you can't get an MSci, think of transitioning to something a bit softer, maybe cosmology, nuclear physics, with some pheno/data component. But note that those fields are still very competitive. If you insist on doing HEP Theory, go ahead, but be warned that most likely you will be jobless.