Transpact.com is FSA Authorised and Regulated as a payment service provider and is an online escrow service, and for GBP transactions charges a fixed fee of only £2.99 per party up to transactions of value £50,000.
Unlike many other escrow services, Transpact.com does not make a percentage charge based on the transaction value.
You also get to specify the conditions that will apply on a transaction by transaction basis with Transpact.com, so you are not shoe-horned into trading under unsuitable conditions.
[Declaration - I work for Transpact.com]