Unlike some other countries, in the UK the supervisors do not have a lot of say in questions related to admissions and funding. Admissions are subject to university rules and these rules are rarely flexible, even if your potential supervisor really strongly supports your application. A student who does not meet minimal requirements is unlikely to be admitted for a program.
Most of the PhD programs in the UK are not funded from your supervisors' budget or their grant, but instead are funded through the Centers for Doctoral Training (which are funded from block grants to the University). Supervisors actually bid for a privilege of supervising a funded PhD student. To progress their career, academics in the UK must successfully supervise a certain number of PhD students. This means that academics are usually very happy to supervise any student who is successful in bringing in the funding. So even if your potential supervisor tells you that they are really happy to supervise you, don't assume that they can and will go an extra mile to bend the university rules for you.