Most UK universities now require you to register as an MPhil student and then ‘upgrade’ to the PhD after a year or so, if you have reached the appropriate level. However, even before the upgrade you can feel free to refer to yourself as a PhD student e.g. on your email signature.
Some people only want to apply for an MPhil, which is one to two years of full-time work (or part-time equivalent), with a shorter thesis than for a PhD. If you are applying for an MPhil only, not a PhD, you should make this explicit on your personal statement and your research proposal.