What is “huge” by UK standards may be tiny by US. When I sold my house in the UK I paid a commission of 1.25% The norm in the US is about 6%
I imagine that at best, you might be able to sue for something like specific performance. I looked into that concept when (as a buyer) I had a signed contract, financing in place, etc. and the seller was having trouble coming up with their closing costs.
All that means (in my layman’s reading) is the court will force the seller to “perform” the contract, in other words, to hand over the property for the amount previously agreed on. Not for free. Though I’m not sure exactly how you’d sue for it since in the example given you wouldn’t have a signed contract to begin with.
Everybody is a member of a protected class. It’s equally illegal to refuse to sell to white people.