Looking at PDX LHR round trip in November and the available fare is $799, Try it with redeeming miles and the fare is 55K miles PLUS $822 so using miles costs the miles and $23 in addition to the listed fare....

Portland, OR (PDX) to London-Heathrow (LHR) Sunday, November 3 Flight BA 266,
Main $307
Refundable Main 27.5k miles + $316
London-Heathrow (LHR) to Portland, OR (PDX) Tuesday, November 12 BA 267
Main $492
Refundable Main 27.5k miles + $506

Booking with BA directly costs $626 r/t

I do note that the redemption fares are refundable but I don't recall ever wanting to get a fare refunded...