Good news, at least today... tomorrow, who knows?
Earlier today I asked a family member to do a single connection speed test on speedtest.net and see what their numbers are (they are also on Lightning Fibre). To my amazement they came back to me with nearly 1 gigabit down. After hearing that I thought I'd give mine another try and again to my amazement it's now nearly a gigabit down too!
Whether it stays this way... well, that's another matter. I hope so. I'll still go ahead with A&A but perhaps I can bond the connections with my usage of BGP, as long as the server at the other end which does the eBGP session can handle that amount of traffic that is. I believe it can.
Their upload was nearer gigabit too while mine isn't quite at that level but I'm not too fussed about that. It's still tons better than I can get via Openreach and it's probably something my end causing a slightly slower upload anyway. I'm just happy that I'm seeing near gigabit download again at last. I've made sure it's not just isolated to that speed test, I'm also seeing similar results with a few other servers and the TBB speed test too.
As I'm curious, I'm going to also see if there's something noticeably different on packet sniffer. I'll edit this post again shortly to mention what I found.
EDIT: The packet sizes are pretty much the same as before, when I was having the slow download speed. No noticeable difference in the packet sniffer output. So, whoever or whatever did something to fix the issue... I don't know what they did at the moment. I hope it doesn't go back to being slow tomorrow though!