Any refund should be based on the premium economy versus coach fare on the date your ticket was purchased. Don't allow them to look at the more expensive coach fares that are available now.

BTW, since you're returning from the EU, EC261 regulations would apply, although a carrier isn't required to do much beyond offering a refund for a schedule change more than two weeks in advance. However, the downgrade rules seem to apply regardless of when the involuntary downgrade is communicated to the customer. Premium economy is considered to be a separate cabin and the refund/compensation is 75% of the cost of the fare only for the downgraded segment, plus refund of any additional taxes assessed for the higher cabin. There's a long, detailed, and definitive discussion of EC261 rules in the BA forum.