I agree that excavation of underground tunnels would be far too deep to uncover worked stones. The old shallow cut and cover of the first steam driven underground railway is quite a different story though. Any finds would have been disregarded, buried or destroyed. There just is no local accessible stone beneath London. Its all clay and the London Stone and other worked stones have been imported. We only know of Roman and Viking carved stones in London (see London Museum) until the medieval period. The rarity of stone makes sarsens rare and special here. Common elsewhere, but in the London basin and elewhere in the general region - sarsens and pudding stones have been valued and used as markers and later as bases for market or preaching crosses eg Kingston and Royston. Bit of a hobby horse of mine.