Your bottle most likely dates to between 1955-1959.
Royal Warrants to Queen Elizabeth II were largely granted during 1955,
and Buchanan's discontinued usage of "Spring-cap" closures in 1959.
An identical bottle was sold at Bonham's Auction in NY on Oct 28-2012
for $166 (including Buyer's Premium), which netted the seller about $120.
(
http://www.bonhams.com/auctions/20459/lot/165/)
Cheers,
Walter C Hurst
wchurst@hotmail.com