Yes, thats correct. Buy sterling on Revolut before you go.
I was in London last year and topped up my Revolut in Sterling. Then just tap/pay as usual in the UK. The only charge was the initial exchange rate when sterling was bought, so avoiding the penal charges that Irish banks charge on sterling purchases, especially Irish debit cards.
Was great for using the tube in London! Just tap at the validators.
If you STG on your account in the STG Revolut wallet it will be recognised when you make a purchase.
Make sure to buy the STG on a weekday though, as Revolut charge a market-adjustment fee if bought at the weekend.