In 2018, I could take an ex-LHR TATL P class fare on UA from experflyer, and estimate the taxes and YQ to be about £500 on top of the base fare shown on expertflyer.
However, doing this today, it wasn't adding up, so I priced out a fare on ITA Matrix, and found that the YQ for my sample booking (LHR-LAX direct) to be £428.

I'm quite sure this used to be about £225-£250, so has it really gone up by that much?!