I was going to say the same as Factor, if it's for you then who knows what might change by the time you get to thinking about a PhD. Would you not need a masters or equivalent first?
You would not normally be expected to do a master's first. There is no point if you have resolved to do a PhD. None of my friends with PhDs have master's degrees, except those from Oxbridge, which is awarded automatically 4 years after graduation and does not involve any study.
I wasn't sure as I know quite a few Drs (mostly science) and most of the ones I can think of did masters although some as part of a 4 year "degree and masters" course like my brother. His wife had a non-UK degree before hers so not the same system. I know two of my exes did masters before their PhDs and some friends so had assumed it was the thing to do.
there is a point to doing a masters...it is a higher level of study than a bachelors degree and prepares you for the horrors that is a phd. most employers and universities would expect that step also. and considering you are thinking of studying a topic that is very different to your bsc...it would be wise to do an msc in a topic that prepares you for a phd in a highly specialised area. otherwise, you will have a disadvantage to other students in the same area (as you will be judged at the same standards as them) and will have absolutely no backgroung knowledge to help you plan your studies effectively.