Electrical issue.

More and more I'm inclined to agree. Here is my latest report:

1. Using a different router port makes no difference. They all behave the same way.
2. Booting with the router unplugged as suggested by sgosnell causes the boot to halt in the same way. But when I plugged the router in and switched on at the mains, the machine started even though the router was still switched off at that point.
3. During the halt, I took the opportunity to go into the UEFI. There's no PXE option nowadays though I'm pretty sure there was one when I bought the machine. I must have deleted it sometime but I can't remember doing so.

Yes, I use dhcpcd. But surely that would only become an issue once Linux is running. This has to be a UEFI problem.

No. My ISP only provides a bare-wires service. I bought that router at a local Maplins store many years ago. It's an all-in-one box; there's no separate modem.

Unfortunately Maplins doesn't exist any more and I have no idea where I could get another one except from somewhere like Amazon. You know I'm phobic about buying online, though I could probably get a friend to do it for me.

What kind of modem is it? If it is ADSL or VDSL it is possible that existing stores have at least one compatible model on the shelf. You can check the catalog online and then pick it up in person.

Now that Maplins is gone, the only store anywhere near me is a Currys/PC World. I just looked at their site and they do sell routers. The ones for "low internet use" are similarly priced to mine (I paid £25 for it if I remember correctly). I assume that "low" here simply means no video streaming.

I think I will drop into the store (if they're still open -- it hardly counts as an essential shop) and see what they have. It never hurts to have a second string to one's bow.

UEFI BIOS are obfuscatory mazes. I'm sure it's still there if it ever was, and you simply haven't re-located its hiding place. There's nothing in a BIOS a user can delete, except by rewriting, recompiling and updating it. There's usually an "Other" boot option that disabling might help.

I admit I didn't search the whole UEFI interface, only the boot sequences. There are three of these: normal boot, error boot (whatever that is) and boot from wake-on-lan. None of the three lists includes PXE but I definitely remember seeing it there earlier. We're talking here about BIOS/UEFI settings stored in nvram, which are deletable by the user, not the program itself.

Boot methods can not be deleted by the user. The user can enable or disable different devices (such as DVD drives, PXE, USB, etc.) and change the order in which the BIOS/UEFI tries them but they cannot be totally deleted. In fact, the bios is responsible for detecting and putting bootable drives in the list.

Sometimes the enable/disable is in a different menu (often deeper) than the boot order so if a device is disabled it would not be a choice in the boot order. Only if the machine has a network device capable of PXE boot (and AFAIK on the motherboard) will that option even be available.