I was really surprised at how expensive it was! Especially as the pound's value at the time was quite a bit higher than today.
It's great that I can commute to work for a £3 round-trip with a change halfway if I choose not to cycle! Of course, that's in South-West London and even then the traffic can get pretty bad. In central it's probably nightmarish to do it regularly.
Especially for lower paid commuters in London. Driving is also a non-starter if you work centrally so it's really your only option.
The only European city I recall being expensive compared to my usual commute was Geneva, Switzerland.
Eastern-bloc tend to be cheaper while the likes of Germany are closer to our prices.
People presume that Berlin is the center of Germany, when in fact it absolutely isn't. Hamburg is a much more wealthy city that Berlin. Here's [1] the ticket prices for Hamburg [1] and here's [2] the ticket prices for London, are you seeing much difference?
[1] https://www.hvv.de/en/tickets/single-day-tickets/overview [2] https://www.visitbritainshop.com/world/london-travelcard/
For Berlin I pay 760€ for a yearly ticket despite not living centrally (AB zone ticket still covers it). London zones 1-2 yearly ticket[1] is £1404 (=1574.45€) again factor of 2 more expensive.
When we lived in Vienna (a wealthy city, probably similar to Munich or Hamburg) transit prices were similar to Berlin (maybe even cheaper).
Not to mention that because London housing is so expensive most "common people" (who aren't living with room mates or are very wealthy) live in what we would consider a suburb rather than the city (in terms of distance from the city center) thus paying fares from further zones (iirc my friends at the time lived in zone 3 & were middle class professionals not paupers and/or with huge families).
From 8 years of living in Vienna and 5 of living in Berlin transit costs were never significant to me & I never lived in the center of either city.
The numbers quoted from the links above line up with my gut feeling when visiting London (my feeling was even more extreme actually, but a few years ago the GBP was worth a lot more € than today).
I haven't tried with a different bank though, so it could be bank specific.
As for recommendations for tourists, Oyster is great, but, there's really no need, a contactless bank card is capped at the same day and week rate as Oyster.
I've been commuting for the last year on my bank card, it's not the most financially efficient way, but it's not a lot more expensive (than a pre-paid pass).
Also bear in mind that a bank card will take ~2 seconds to scan at most stations' gates, versus milliseconds for an Oyster. It might seem trivial but at London rush hour it's an annoyance for you and everyone else.
One thing people also don't know is that at most stations, your card won't scan if you're standing between the gates, step back and scan or you'll just hold everyone up until you give up and walk away.
It's designed this way, I imagine, so that you don't scan while someone is still paying through the gate and have it close on you before you pass through.