Yeah, I don't feel great about this at all. It will make me treasure the current season even more than I would have normally.
In the US, UK seasons 4-6 (US seasons 1-3) are available through the PBS app's Passport service. You have to be a member, but at $5/month it's not a terrible deal.
I have NEVER been able to find UK season 3, and it haunts me.
I've mentioned this before, but for those looking for a bump to get them through, I would recommend the Great Australian Bake-Off, series 2. Avoid series 1: poorly-chosen judges and hosts make it leaden... which doesn't make me feel any better about the upcoming changes to GBBO. Sigh.