At the moment Post-Graduate Research students (PhD or equivalent) are treated as not working when they apply for Universal Credit (UC) .This means for every pound of their Research stipend their UC award is reduced by a pound. It also means that they cannot claim other costs, eg childcare.
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Post Graduate students were seen as essential workers during the pandemic. However, the Department of Work and Pensions deems them to be students and not working. Thus while they receive a stipend for their efforts, this comes straight off any entitlement to Universal Credit. This mainly affects post-grad students with children. If their stipend is regarded as payment for work, they would not lose all their stipend and they could also claim travel expenses and 85% of their childcare costs.