Is just 'being interested in the topic' a bad reason to do a PhD?
To be a bit snarky towards your professor, I don't think that we can truly answer this question until we have some sort of passion-ometer that can distinguish degrees of caring.  How much interest is "enough" is just not quantifiable.

That said, there really does seem to be a qualitative difference in how people do in a Ph.D. based on their degree of motivation to be in the program.  There are lots of good things to do with one's life that are much more short-term rewarding than doing a Ph.D.  Thus, people who enter a Ph.D. program just because it seems like a reasonable next thing to do with their lives are likely to run into difficulty and end up exiting to one of those other nice alternatives.  People who are having trouble but do not have alternatives and feel trapped in their program (e.g., due to visa issues) are at a strong risk for harm to themselves or others, up to and including suicide or violence.

But passion for a particular topic isn't a make or break.  I think the key is whether a person finds the ongoing struggle of research rewarding.  If you find that you have a taste for that, a Ph.D. is a good idea; if you don't, it's probably a bad idea.  

And it's difficult to really know until you've really engaged with some big research or engineering projects.  If you have, you probably know if you've enjoyed them enough to make a life out of it, whether you stick with a particular topic or end up shifting over time (as most active researchers do eventually, as their original topics either succeed or hit dead ends).  If you haven't ever really engaged with research before, I would recommend getting involved in some research first before making the longer commitment to a Ph.D.---although a 3-year UK Ph.D. is short and focused enough that such a preface might not be necessary.