Essence Institute B.V. How you might wanna send an international transfer depends on two things: the destination country and the currency of the bank account that you're sending money to. Of course, in general most bank accounts in a certain country will be denoted in that country's national currency, but that's not always the case. Also, some banks that you send money to will still accept money even if it has been sent in a different currency than the receiver's bank account (they're able to convert it), and some other banks won't be able to do that. These are the main factors for an international payment.
Inside the bunq app you can find multiple ways to send an international transfer, and while they can seem redundant sometimes, because of the aforementioned reasons, that's not always the case. It'd be hard to sum up everything in a single comment here, but to give you an overview to start with:
There are Local Currency accounts available at bunq. They can have different currencies, but the list is currently still rather small, but will probably expand soon. These accounts are all UK bank accounts (because they work through a partnership with Currencycloud from London), but of course not limited to GBP. Depending on the currency of the account, you'll be able to send payments via both SWIFT and the respective country's domestic bank network. Most of your customers would probably prefer you to use the latter, and it is also generally a cheaper option for you. More info on pricing in the Pricing Sheet.
The service Wise is also integrated into the bunq app, making it easy to use. To use that, instead of opening a new account, you just click on "Pay", then Select currency and then tap on the blue text "Currency not listed? Use Wise". Disregard the fact that it says "Currency not listed", because it also works for the currencies that are listed above on that screen. It actually also works for Euro payments. Now in general you want to use SEPA transfers when sending international Euro payments, but if you want to send Euros to a non-SEPA country, this is a handy option. (Montenegro is a good example, they even use the Euro as their official currency.) With Wise, you will most often be able to make a domestic-type transfer, but SWIFT is also an option if needed. Searching for your currency and/or country on the Wise help pages is a shrewd thing to do here.
Last but not least, transfers denominated in Euro to a country inside the SEPA, you should of course just send as a regular transfer, just like when you send money to another bank account in The Netherlands.