Goodness! That's a huge question! I don't think the full/part-time issue would be a major concern when it comes to job-hunting & application, unless you went over the time given for completion at your institution.
The competitiveness of the field particularly in a big subject such as History is. I've been on selection panels for various History departments (I have a first degree in History among other things) and it's usual to receive up to a hundred applications for an entry-level Lecturer post.
It helps to keep what I'm going to say below in line with a notion of 'career-age' -- that is the equivalent full-time years clocked up as a post-graduate.
It helps if you have had AHRC/ESRC funding for your PhD -- this shows you are already outstanding in your field and career-age.
A PhD alone is not enough, but you will not get a job in a History department without a PhD in the current competitive climate. Well, certainly not at a decent university.
If you seriously want to work in academia, you will need to be mobile. It's not a matter of picking up a full-time job at your nearest "office." Seriously, some people think that's how universities work!
You will need to have your PhD, and without an over long period of candidature ie if it's part-time, then you'll need to complete within your institution's part-time registration period -- generally 6 years with an option of a period of 'writing up' when you receive little or no supervision, or an extension on application because of other factors (never ever take this for granted!).
You will need some HE teaching experience. A good department will try to offer some of its 2nd year PhD candidates hourly-paid teaching as GTAs, but this isn't universal, and sometimes the PhD candidates are just not suitable or not equipped for the teaching the Department needs. At my current institution, it is subject to satisfactory progress, and that progress must be maintained for us to employ them,. So you add a term-time teaching load to your research load.
You will need to demonstrate a trajectory of post-doc research: conference papers while a PhD candidate, publication of an essay or two, and a serious plan for your first monograph, if not a contract.
You should have serious, workable, peer-reviewable plans for grant or funding applications (for example, AHRC Early Career Fellowship, British Academy Post-Doctoral Fellowship, Leverhulme Early Career)
You will be competing against applicants for jobs who have their first book.
Post-PhD, you should expect nowadays to have to spend at least one year, if not two or even 3, doing short-term contracts, or hourly-paid teaching, or working as a Post-Doc Fellow on other people's research grants. During this time you would be expected to publish from your PhD. If you don't keep your hand in in these ways (teaching and research) you will not be competitive in the job market against 'fresh' PhDs with book contracts.
And these are the basic things now, only 6 months into the withdrawal of 80% of public funding for university teaching, and the consequent £9,000 fees regime. And 10 months away from REF 2014. Each year UK HEIs go into undiscovered territory, but the next 18 months will be more unknown unknowns than ever before.
I know there'll be other posters to say it's different at their place: these are general guidelines drawn from my working in 3 research-led universities (Russell Group).
This is why most good academics are workaholics: standards are extremely high and exacting, but we do it because we're all actually quite good at it, and we love it. This is also why £9,000 a year is a very cheap education.