PhD in theoretical physics: good or bad for engineering jobs?
I would say that strongly depends on what you do in the HEP PhD. 

If it is more like pen and paper theory I would say this is not very interesting for industry. Perhaps even in the contrary as you might seem too "academic". Pen and paper theory jobs do not really exist in industry.

It is more the big data processing and computer simulation related skills what is relevant. The tools (python and methods) you use in your MPhys research sounds exactly what some companies are also using internally. 

Industry considers hiring people from adjacent fields, when demand is much higher than the supply for certain jobs. Currently I see that this is the case for "data scientists". So I would say now your chances are good to find a job there as many companies are now exploring big data applications. They are scared by the success of Google, Facebook, etc.. and do not want them to take over an important part of their business. So many are investing here. Industry also has these jobs so it is not just finance. It is very diverse. One gains some feeling for it when one looks at current job offers (which is what I did, so I have no deeper experience than that). However, you have to keep in mind that the situation may be different is in a few years when you might have to give up your academic career (hopefully not). 

A PhD is liked for certain types of jobs even in industry. As the PhD is teaching more a generic mindset (-> how to work scientifically,/thoroughly, know how to tackle big, unsolved problems) this is considered useful in some industrial jobs. Appropriate job descriptions would perhaps have names with "scientist" in it, like "data scientist", "research scientist" and so, not so much the "engineer" type of jobs. Sometimes PhDs are also found in sales/marketing, when a product needs deep explanation, training (and the customers are themselves PhDs) or must be customized to the users needs (and this is not straightforward).

Note: Even though I have worked in the US, I a most familiar with the German system. Nevertheless I hope the answer helps somehow.