Can someone at giffgaff (wont hold my breath for that one lol) explain to me whereas I now get no signal at all using my giffgaff SIM because the "local" mast is obviously down again, but using my new Lycamobile SIM (also runs on O2) I get one or two bars. This is from standing outside, I get no signal indoors. So not only do you have lower priority on the network compared to O2 on giffgaff, you also have a lesser signal strength, that makes no sense to me.
If there are other masts in your area, a phone will try to connect to any mast it can reach but those masts will have extra loading because of the mast that is down.
In overload conditions, it's not possible to predict which (if any) mast you connect to because the loading on masts will vary continuously as other users connect or disconnect.
So, you might get stronger or weaker signals, or no connection at all -- you would have to perform a suffiiciently large number of disconnections and reconnections with both sims to get any meaningful conclusion whether you were just lucky in connecting at precisely the right time or whether there is genuinely any priority allocation.
Thanks @harrrrrry. Bear in mind though I'm on a little island out in the sticks - there are very few phone masts here at all! The "local" one which is down I refer to is about 13 miles away, and that is also behind a hill. So now the weak signal I'm getting is now from even farther away. Three is still working as normal I'm using it now - mobile broadband (outside antenna), and the Three SIM in my phone is working too (outside).