I want to close this thread off with a final outcome. Just over a week ago I changed ISP from Sky to Aquiss, I simply changed my (OPNsense) router from WAN DCHP to PPPoE for the new connection, nothing else has changed whatsoever, and after a week of hammering it (over 2TB in bandwidth) I have not encountered a single issue or slowdown.
I cannot explain what was going on with Sky, I'm not entirely sure it's an intentional think from them, maybe an issue with their setup somewhere along the line, who knows. But alas I finally do know it was indeed the ISP causing the problem. I do find it strange how few people are reporting this problem as I'm sure I cannot be the only one using the connection like I was, but there are a handful of others that have complained about it.
Sure is good being able to use my connection at full tilt without having to worry about it bringing the internet to a halt, which as anyone will know, has a very poor WAF.