Next time try Pingit, it’s free.
However, exchange rate is terrible. For example, when revolut is 1.1396, Pingit 1.1058. Fee is about 3%.
You can also use Fire.com. Exchange euros at revolut and send it to Fire (two working days), free; Use Fire to do SEPA to Coinbase/Kranken (Same day if sent before 2:00 pm), free.
If this takes too long for you, transfer money from Barclays to Fire (few hours via bank transfer, instant with debit card but 200 pounds daily limit), free; exchange at Fire at a fee of 1.25%, instant; then SEPA, free.
Another solution from Monese. Monese is launching euro account with your personal IBAN. When this launches, you will be able to top up GBP by bank transfer (typically 2 hours), convert to EUR at a fee of 0.05%, do SEPA. Dont know how long SEPA takes since it is not out yet.
Again, wish Monzo is heading to unique IBAN.