Hello, this is a very valid question.
Your main currency is EUR and you added GBP as a foreign currency.
In case you did not exchange any GBP and you pay in GBP while you travel, what happens is that it will convert your GBP to EUR with the spot rate (Revolut takes no commission on a week day).
In case you have a Revolut GBP balance, when you pay in GBP it will deduct from your GBP balance (assuming you have enough balance).
Which one is better? Well you can never predict the exchange rate, so you never can tell.
However, there is one major benefit of exchanging EUR to GBP in advance is that you can avoid the 0.5% Revolut charge during week-ends, so it is better to use on the week-end some GBP exchanged on the previous Friday (assuming no exchange rate fluctuation between Friday and Saturday/Sunday).