Where is best to sell a collection of pure gold limited edition proof coins?

[Jake Ibbo]

Hi there, 

I am new to this forum. 

Im enquiring on behalf of my step father whom has a collection of limited edition proof pure gold coins of significant value, he is now well into retirement and would like to sell these coins in order to free up some capital In order to make the most of his retirement. 

He had a company come to value the coins however would only offer him the spot price for the weight of the gold (around 15,000 pounds), however we believe them to be worth considerably more as they are limited edition in perfect condition and never been touched and we have all the boxes and authenticity paperwork still. 

The List of the coins are as follows, 

PCGS-graded 2016 '5-Portraits 90th Birthday' 1oz Proof Limited Edition

Longest Reigning Monarch Proof Sovereign Set of 3 (2g, 4g, 8g)

PCGS-graded '5-Portraits' 1oz Proof Satin Finish Limited Edition

PCGS-graded 'Remembrance Poppy' 1oz Proof Gold Limited Edition

PCGS graded 2006 American Buffalo $50 MS70

PCGS graded 2015 Proof Sovereign £1 'Long Reign' Special Edition

PCGS-graded 2015 Kangaroo MS70

PCGS-graded 2015 Philharmonic MS69

Coronation Twin Set - Set of 2 x Proof Full Sovereigns

Would anyone be able to provide me with some advise on where would be the best place to sell these coins please? 

Any help would be greatly appreciated.

Thank you 

Jake