I recently interviewed for an academic job (unsuccessfully!) and for the second time now the issue came up of uk phd students having too narrow expertise. If you are only researching for your thesis and not taking courses, does that not make you incapable of teaching more broadly? This is the kind of issue you may have to deal with if you do your phd in the UK.
Here are some ways to deal with this: Before your phd: choose a thesis topic that is multi-topical. Obviously it has to be manageable for your research project, but think about gaining some expertise in two areas. For example, I have a friend who compared Josephus and Paul on relationship and attitude to the Roman empire. I have another colleague who is comparing Colossians and 1 Peter. I think this is very wise. These kinds of theses are often very interesting to read.
During your phd: audit courses in your university (post-grad courses). At Durham, I audited Paul and His Interpreters (John Barclay), the Septuagint with a focus on Tobit (Loren Stuckenbruck), Social-Scientific Criticism (Stephen Barton), and the Gospel of Mark (William Telford). Also, try to do some teaching, even if only as an assistant. I TA’d for undergrad courses: intro to NT and NT theology. Even better, seek opportunities to teach. Perhaps the university will let you teach Greek. If the University has a theological hall or college, see if they have needs. I taught Greek at Cranmer Hall, an Anglican ministerial training centre under the umbrella of Durham Univ. Also, do book reviews and broaden your horizons with the books you review (I have done several posts on how to do book reviews; do a search on my blog to see them or look at the Guide for Researchers under the Pages above).
Other options: for your thesis, if it is narrowly focused on one thing (like Paul’s view of such and such), do one chapter on something related. I had contemplated doing this, with one chapter on Philo and another on 1 Peter. Unfortunately, it didn’t work out.
During the interview for a job: If you get the ‘too narrow’ quest, inform or remind the interviewers (if you are interviewing outside the UK) about the New Testament seminar – a regularly scheduled event (almost weekly at Durham) where scholars come and test their ideas. The topics are varied and sometimes very productive and significant conversations take place. I wish this tradition made it across the pond!
In any case, keep in mind that many potential employers will be small liberal arts colleges and they care more about teaching and teaching experience in their candidates than their research potential (in my experience). This takes forethought to get that experience and also some on-your-feet thinking during the interview. You need to remember that, while finishing your thesis is a necessity, it is not the only thing you need to be doing during your phd. If you want a job, especially in this economic climate, the bar has been significantly raised.