Thought that I would give an update and my personal solution.
The M247 servers were continuing to give me problems. They would be fine for days or hours, then all of a sudden, websites would take ages to load and speeds would drop. This would occur randomly. This would resolve after a few minutes. Probably happening more often, but of course I only noticed when I was at the machine. Speed tests show the speed high to start off with and then the speed would gradually drop as though the server (m247) was either suddenly overloaded or there was some kind of rate limiting going on.
For the last month I have been using Netherlands servers. I am seeing none of the problems mentioned above. My ping times to the Netherlands servers are generally lower than UK servers too. 10 or 11 ms compared to about 20 ms. Websites feel faster to load and I am getting consistent results on speed tests no matter what time of day.
I do not know what is wrong with the M247 servers. It seems to coincide when they moved data centre last year. My guess would be that it was bad peering with my ISP but @Frozzeled was experiencing issues too, who is using a completely different ISP to me. My suspicion is that there is some kind of rate limiting happening somewhere in M247's network. I don't think that it is intentional, probably something misconfigured somewhere. Again if there were another provider in the UK I could do a like for like comparison, but unfortunately there isn't. For now I will be sticking with the Netherlands until Eddie 2.23.2 or higher is stable and I can try the MTU change.