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SEPA Bank transfer to Kraken is free on all countries that support it (there might be a British equivalent), and their exchange rates are very low as they don’t keep a pound or two on each transaction. I can exchange 5 euros to any crypto and the fees are 0.008 euros for this amount.
If you want to move them to your Ledger, you have to pay the transaction fees which are not related to the exchange service. For example ethereum transaction fees vary from 4-25usd (they fluctuate every minute, every hour, always), which means you have to pay this amount no matter if you want to move 1£ worth of ethereum or 1 million from your exchange to your ledger wallet. But other networks have very low fees and are better for day to day operations in low budgets, for example Litecoin (LTC) transaction fees are $0.03-0.04 per transaction. There are even lower transaction fees on some other coins but I don’t trust them that much.