This appears to be free for consumers at least. Some businesses may be charged. Amounts are for £1-£300 and there is a maximum of £5000 a day. You do not actually need a uk mobile to receive just a uk bank account, so people abroad could take payments relatively easily.
Small clarification on limits - you can send a maximum of £300 a day, and receive a maximum of £5000 a day
Are there any uk banks that have a decent API so you could see these transfers and associate it with a client automatically?