That sounds like a Carrier Profile job tbf. And if not, it'll be steering by O2 themselves (I think a bit like how I keep getting put back onto AT&T after manually selecting and registering to a network)
Agree, steering can result in very weird behaviour. Eg I've often been able to get alternative networks even on the notorious Vodafone but only with luck (and often being in rural areas on my trips)! O2 will steer heavily to Movistar.
Talk Home will have:
France:
Orange 5G
Free 5G
Bouygues 4G
No SFR
Spain:
Movistar 5G
Orange 5G
Xfera Yoigo 4G
No Vodafone
Andorra:
Telecom Andorra Mobiland 4G
These are all off the top of my head so don't take it gospel, but that's what you can expect
I used them as a second SIM about a year ago on a trip through Slovenia, Austria, Italy, Switzerland (at the time it was included, kinda - data worked, but calls and texts didn't!), Leitchtenstein, France, Hungary, Romania and Bulgaria.
I didn't do thorough testing, but at that time it was O2 - SZ in Slovakia (4G only), A1 Austria (5G), TIM Italy (5G) and Illiad (not sure, wasn't in a 5G area), FL1 Leitchtenstein (4G only - no access to SwisscomFL or 7acht), Swisscom (5G) and I
think Sunrise Switzerland, Orange Romania (4G - no 5G), Yettel Hungary (not sure if 5G), Yettel and Vivacom both 5G in Bulgaria (definitely couldn't get A1). This isn't an exclusive list at all.
Also I remember my Lebara SIM was very confused in Leitchtenstein, worked fine on FL1 up to 4G but whilst it would connect to SwisscomFL, it acted as if I had no credit (perhaps as if was in Switzerland, where roaming isn't included - and yes I was definitely on SwisscomFL NOT the Swiss network!)
At that point I did get 5G on Bouygues. Also had access to Free and Orange, both 5G. I don't remember about SFR - coverage wasn't good for them where I was (even Free was better! - but as you can see I was struggling even on those!)