Anything bought in the London area will be overpriced and anything with Holland & Holland and Purdey on it will be too.
How did you arrange the trip around Hollands?  I tend to avoid London but will go there for a specific purpose and then out again.  A trip around there might tempt me to visit the capital.  Went around Greeners set up last month.  It didn't take long as it now only employs just two people.  I did manage to buy a 1976 H & H catalogue the other day.  It was in a slip case with another booklet giving a brief history of the firm and with quite a few photos and bits not in Dallas's book.  There was also a 1936 catalogue but the price was too high.  Also got the 1985 catalogue at the same time.
One photo shows a fairly substantial set up by the early 1890's  They seem to start recording serial numbers of their own guns from around 1856.  Lagopus.....