It’s a tiny gripe from an otherwise successful trip, but turns out using Oyster (or contactless) on London tube and trains is always cheaper EXCEPT when travelling to Gatwick in the rush hour, and then it’s £4 more expensive. I discovered that was the case for morning rush hour on my last trip, this trip evening rush hour (post journey of course). So, now you all know! And yes, I still use an Oyster card, because I’m a retro tube traveller.