FTTP is a shared medium, that's why nobody can guarantee the speeds. The total available on an Openreach FTTP connection is 2.4Gbps download (1.2Gbps upload) divided between up to 32 users on one splitter, which goes back to one OLT port in the exchange. So when someone like you is downloading at 950Mbps continuously, that makes less bandwidth available for other people in your area - and vice versa.
"High radial counts" is totally meaningless. They might have said something about RADIUS I guess, which is what's used to authenticate a session, but once your session is established it has no effect on the bandwidth delivered to you.
You're quite right that as well as FTTP congestion in the Openreach last mile, there can be congestion in the ISP's own network out to the Internet. As for ISPs buying less capacity than they need, it depends what you mean by "need". Nobody buys bandwidth equal to the number of customers they have multiplied by the peak bandwidth each customer has bought: that would be mad. They monitor their usage, and increase their capacity as demand requires.
Vodafone *is* cheap, and you may find that if you need to talk to their customer service department, it's not great. But you could say the same about BT.
BTW, at 500GB per month, you're not a heavy user by any stretch. In the OFCOM market report from July 2022, the *average* household usage on fixed broadband connections was 453GB. AAISP Home::1 has two plans: normal usage has a 1TB/month cap, and high usage has a 10TB/month cap. (This is probably the best home ISP you can buy)
Downloading 20GB at 900M takes about 3 minutes. At 300M it takes about 9 minutes. Either way you're going to need to make a cup of tea and have some biscuits
Personally, I would take 300M from a high-end provider with good customer service and static IPv4/IPv6 over 900M from Vodafone or BT. But it's down to each person's individual needs, preference and risk profile.
Edited by candlerb (Wed 11-Oct-23 13:57:32)