I think this mostly applies to the EU and Northern Ireland, but since the rule hasn't been adjusted for inflation the threshold in number of gold coins we can carry with us, without triggering the rule, has been silently dropping.
This might seem obvious since monopoly money has lost value over time, but I found it more surprising once I got the data (please double check it):
Year
Gold Price ($/oz)
EUR/USD
Gold Price (€/oz)
Gold Price (€/g)
Sovereigns (€10,000)
Britannias (€10,000)
2007
656.50
1.36
481.48
15.48
88.22
20.77
2008
942.50
1.59
593.59
19.08
71.56
16.85
2009
941.00
1.41
665.45
21.39
63.83
15.03
2010
1206.30
1.25
964.46
31.01
44.04
10.37
2011
1482.30
1.45
1022.49
32.87
41.54
9.78
2012
1597.20
1.27
1262.09
40.58
33.66
7.92
2013
1255.90
1.30
965.03
31.03
44.02
10.36
2014
1326.40
1.37
968.63
31.14
43.85
10.32
2015
1169.00
1.11
1049.84
33.75
40.46
9.53
2016
1336.70
1.11
1203.58
38.70
35.29
8.31
2017
1217.90
1.14
1066.17
34.28
39.84
9.38
2018
1239.80
1.17
1061.60
34.13
40.01
9.42
2019
1385.60
1.14
1219.19
39.20
34.84
8.20
2020
1773.20
1.12
1578.15
50.74
26.92
6.34
2021
1775.90
1.19
1497.79
48.16
28.36
6.68
2022
1798.90
1.05
1716.89
55.20
24.74
5.82
2023
1921.70
1.09
1761.81
56.64
24.11
5.68
Right, so in 2007 (ChatGPT tells me this is when the rule was created) you could bring with you 20 1oz gold Britannias without having to declare anything. Today that number is around 5.
If you plot the thresholds, that is the number of coins you can travel with without doing the customs declaration:
I'm afraid the conclusion from this is that, unless the rule is adjusted, there will be a time when we'll have to declare even a single Britannia. Perhaps something worth thinking about.