Is this in the same location without moving anywhere?

There are two O2 cells here, but one doesn't seem to have HSPA. Because we live in between the two, if we try to use it, the modem will keep swapping between them because both have borderline signal strength.

So you could see this if ordinarily there are 3 available cells and the one nearest you is broken.

I don't know much about iPhones, but I do have one - a software update in the past rendered the Wi-Fi useless (it only works when the phone is within five feet of the router) so that got abandoned. Have you installed any software updates on the phone? Might it be a known issue with the handset?

Suggest you give 3 a call - as in the case above they might be able to give you an update on the service.

Here, it hasn't gone down for as long as I remember, it stays on HSPA all the time so it doesn't seem to be a widespread issue.