Regulation 261/2004 provides for a 75% refund of the fare paid in case of involuntary downgrading. No exceptions, no extraordinary circumstance defence.
While this may not be exactly what happened but it certainly a starting point for superq7's negotiations. Why not remind Thompson of their obligations under EU regulation. If they demure contact the UK's National Enforcement Body, the CAA, and ask for their views. I doubt they would be sympathetic over an airline promising something, and not delivering it.
Thompson, stop lying to your customers. It comes back to bite you.