All things being equal, wouldn't that greatly reduce the benefit amount per person?
I don't know about the UK, but in the US a substantial portion of the budget is spent on bureaucrats ("caseworkers" and the like) who would be unnecessary with UBI.
According to the radio (BBC), a substantial portion of the budget is going to contractors who've started rejecting perfectly good applications.
Administration costs go way down. We're spending a ton on making sure that only the "right" people get benefits and policing the poor.