Watch List notes:

BRITISH COINS, MILLED GOLD SOVEREIGNS, Elizabeth II, Gold Proof Sovereigns (3), 2003, 2004, 2006, fourth crowned portrait right, engraved by Ian Rank Broadley, IRB raised below head, rev St George slaying dragon with sword, helmet with streamer, date in exergue (Bentley -; S 4430). All practically as struck, without boxes or certificates. (3)
Numbers issued 12,433; 10,175; 9,195 respectively, including those included in gold proof sets
The 2006 dated Sovereign has a slightly taller date configuration than the earlier pieces.

Estimate: £550-650