Often, doctoral candidates take some additional work within the university - a teaching assistant role, or demonstration work in student labs. This can be very good experience, especially if you seek an academic career, as well as providing additional cash. When you're looking at possible institutions at which to undertake a doctorate, you might want to ask about opportunities like this and whether they would help you to access them.
Bear in mind that your funder or university degree programme may have rules that restrict the number of hours of paid work you can do. Even if your funder and doctoral programme allow you to do paid work, international candidates from outside the European Economic Area (EEA) and Switzerland and registered as full-time students will need to know the restrictions placed on them by their visa. For example, a Tier 4 student visa currently allows you to work up to 20 hours per week during your programme. On the other hand a full-time work placement may be allowed where it is a formal part of your programme. If you are of UK, EEA or Swiss nationality, these restrictions do not apply.
See further information on applying for a visa for doctoral research in the UK.