Go Cardless is a UK-based direct debit service.
It is free to open an account and they charge just 1% of the total transaction fee + 20p per transaction for UK and EU payments and 2% of the total transaction fee + 20p per transaction for international payments. Compare to Paypal and other payment processors this is a VERY cost-effective way of taking payments.
This would suit you if you take recurring payments or you bill your clients different amounts regularly for work you may complete for them. We use this a lot as it is ideal for what we do.
There ARE plugins available to include this as a payment method for WordPress.
This company are used by HM Government, The Guardian, Trip Advisor, Thomas Cook and Virgin so you KNOW that they are reputable.
They also support the export of data to QuickBooks.