Some progress.
Yesterday evening I received a phonecall from my complaint with a nice CS agent where the customer service agent was also irritated at the inability of their systems to allow them to apply the upload speed add-on at the correct price, so we went in a bit of a loop of him trying to add it, he sent the case through to a manager as he said I can have it - it's just finding who can add it for the correct price.
This morning I've had a call from the CS agent the original CS agent sent my complaint through to, and he's tried to add it but it keeps saying £16, so he's sent it through to a department who is currently manually adding it to everyones account who has requested it. Apparently they're applying the £16 and applying a £10 credit per month to make it £6 - seems a backwards way to do it rather than fix the system so it's £6 as default - but seems it's their interim solution. He's nudged my case forward with that team and asked them to move it to the top of the queue since I have a open complaint. So

I will have it by Monday.