I don't believe Boosey and Hawkes never sold heavy valve bottom caps, and I believe these were a prototype recovered from the factory when it was sold in 2003. I think these are quite unique and probably very rare.
Selling as a set of three but often
Dimension are in photos and weight is 26 grams
I have one set of these and two other sets of (17g) caps for sale
On a search, two UK-based music instrument suppliers are selling a single heavy valve cap for £75.99 and the other at £88.99