During your studies you will almost certainly go to some conferences. This is part of the working life of research academics, and you should expect to become familiar with them as part of your research training. So discuss attendance with your supervisor (and Thesis Group), and include them in your training plans.
Typically, you will be going to conferences in order to present your own work, having written a paper and had it accepted for publication and/or presentation at the conference. Writing papers and presenting them is not an explicitly required part of your research degree, but it is a key skill in effective research. In addition, your thesis will look more solid to the examiners if it is backed up by papers of yours that have been accepted for conferences.
More generally, attending conferences (or workshops, or other events such as summer schools) is a valuable opportunity to meet other people in your own research area, listen to talks that tell you about current work in the area, and get used to discussing your work with others.
You will need to obtain money to fund your attendance at the conferences (another key skill in effective research!). Your first step should always be to discuss funding with your supervisor, who may already be able to pay from money under his or her control. If you have a studentship attached to a research project or grant, that may include an amount for research costs, including travel.
Some funding from the School is available from the School's Research Committee; your supervisor or the Research Secretary can tell you more about how to apply. Online application forms are available at the Research Committee website. Currently, if you do not have other sources of funding, you can expect to be funded to attend one conference up to £1000, plus a summer school or training event up to £500. There is a degree of flexibility, depending on the current budget, but you would need to make a good case for further funding.
Remember that a key part of the justification, certainly for any funding from the School, will usually be value for money. An expensive trip will need greater justification in terms of benefits, whether to your studies or to the School. Benefit to the School is normally counted in published papers, preferably in high-quality and well regarded venues. You will also need to get a letter (or email) of support from your supervisor.
Please remember, that when leaving the UK (for example, to attend a conference), international students need to complete an authorised absence form in advance, and then report back to the Research Students Administrator on their return.