I love how you are so "well you cocked it up, stop whining" as far as I understood we sent in the required documentation, their request was for additional documents.
I understand that you want answers from UKVI and it's frustrating not to get any news. I've seen a lot of people stressing out about their visa applications over the last 8.5 years and from seeing their experiences, the best advice I can give is to try to put the visa out of your mind and not panic about how long it's taking because all that's going to do is stress you out unnecessarily.
In terms of the documents, there are 3 definite requirements you must meet for the visa and a very specific set of documents you must send for each one. These are all listed clearly in the documentation on the UKVI website.
The most important requirements are the accommodation and finances and without these documents, Sheffield cannot process the visa. UKVI didn't ask for extra documents that were additional to the requirements, they emailed you to request the REQUIRED documents that you hadn't included.
If your original application package had included all the required documents, they wouldn't have had to email you at all and your visa probably would have been processed by now.
They don't email for additional, non-required documents because there's no need to - the visa is 'tick-box'... if they can go through your documents and tick them all off the list, the visa will be granted...there's no need to see extra documents on top of the requirements.
Having said that, even with the delays in your application, you're still only at 24 business days since your application was acknowledged. The average processing time for settlement visas in June ws 30 business days (July stats haven't been published yet), which means you're still within normal processing times.
I've complied very quickly. I've paid for the priority upgrade, the least they could do was acknowledge the documents had been received. Horrible system.
Yes, it's frustrating, but unfortunately that's how it is and it's unlikely to change any time soon. Some people get email acknowledgements and others don't.
The priority upgrade will ONLY get your application moved to the front of the processing line so you can skip the queue. That's it. Once it is in the hands of the entry clearance officer, all bets are off and they can't guarantee any kind of processing time... it takes as long as it takes to process, whether that's a couple of days or a couple of months.
Since your application was missing several mandatory documents and they had to request them before they could start processing the visa, it's going to take longer to process than an application that contained all the documents when it first arrived.