Delaying in getting official diploma and starting FT

Hi all,

I converted my SA to a FT position at a BB in London. My courses are officially done this December and I will get a transcript around February. On top of that, my thesis will be defended by May, but my official diploma will only be issued in October. My FT contract assumes obtaining a bachelor’s by mid June. At that time, I will have met all graduation criteria, but no official diploma yet. HR said they would accept proof of degree, but not sure what is meant by that exactly. Anyone had a similar issue and can share how serious it was? I would greatly appreciate any inputs!