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 in commission. What’s more, this is capped at £2 per transaction so this is a VERY cost effective way of taking payments.
This would suit you if you take recurring payments or you bill your client’s 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 export of data to QuickBooks.