I would recommend you try the SFO-LHR or LAX-LHR runs that provide 10,000+ miles for the roundtrip, which you can double by paying an extra $100. Of course, you have to get to the west coast from DEN, or alternatively you could try to connect via ORD which would provide almost the same mileage.

I think it will be hard to top $300 for 20,000 status miles.