Update:
I’ve now charged 3000CHF to my Revolut account. On the same day, I did a small purchase in the Windows store with the same UBS preapaid card. This is my bank statement:
One can see that the 1.75% surcharge appeared for the Windows store purchase but the Revolut top-up was completely free.
Either it’s a UBS bug or somehow Revolut has its payment processed in Switzerland (despite the GBR country code).
In either case, currently the scenario of topping up with a Swiss debit card, converting with Revolut then transferring to an EUR account is better than using Transferwise.