We just spent the month of September in London. Most fortunate to have great weather and wonderful experiences.
We rode the tube everywhere using Oyster (not visitors version). I was surprised to see we spent about 140 GBP each. That seemed like a lot, so I checked the Paris Metro. The unlimited card (Navigo) costs 76 euro. We used it a few years ago and found the Paris Metro about the same as the tube, except there are not as many escalators in Paris and more long stair climbs which are tough on those over 70 like us. So London is more than twice as costly as Paris.
Did I overlook some better card than Oyster? (I believe Londoners have annual options that are cheaper.)
By the way, I wouldn't change anything if I had known how the cost would add up. And London tube riders are more courteous than in Paris. We were surprised at how often we were offered a seat by younger folks. Very nice.