You'd open up a trading account, deposit funds from one country to it (generally free in EU/UK, can be done with a simple bank transfer, though can take 1-3 business days). Rather than the common use case of a brokerage account to buy stocks/invest, you place an order into their IdealPro order book they run to buy the currency you want (might have to jump through 2 trades, they don't run every single pair). Generally this is tailored at larger investors, so anything under $25k is subject to some additional fees due to being a small size (https://ibkr.info/node/1459), but still significantly less than the 0.35% TransferWise charges. The costs for you are commission (<0.2bps), potentially small size fee (1-2 bps), and crossing the bid/ask spread, which is ~0 for currencies). When that order has been filled you now have the desired currency in your account and you can transfer it back out (potentially to a different country or different account).