Yikes ... this is brutal. Is there just the one phone number to call? I'd love if I could send them additional documentation somehow that may help
whether through a family member in London bringing it to them physically or me emailing it ...
The only number to call is the third party 'helpline'... but it's anything but a helpline. They are just a call centre nowhere near Sheffield, who have no access to your application and are known for giving incorrect and inaccurate advice (like telling someone a decision had been made on their visa, when it hadn't even been looked at yet).
It is not possible to add any extra documents after your application has arrived in Sheffield unless they contact you specifically to ask you for it, in which case, you will be given a deadline and a specific contact to address it to.
It only takes 1-2 hours to actually process a visa application once they pick it up, so 99.9% of the 'processing time' is just your application waiting in line for someone to pick it up and look at it.
So, all you can do is wait, I'm afraid.
Having said that, someone else on the forum who applied non-priority got their 'documents received' email from Sheffield on March 5th and they received their 'decision made' email yesterday... on business day 64... so since you got your email on March 6th (and are currently on BD 63), you could be very close to getting a decision now.