Always worth asking at outstations, though not in London in my experience. My list so far where I have asked, or proactively been offered cash u/gs are GRU, EZE, MEX, JFK, SFO and as I recall, PVG. They were all either WT+ to CW or CW to F... and were all gratefully taken up as the cost was between 300 and 600 USD and very reasonable I thought.

HL