I started an earlier poll about the 2 pound coin:
When should the first circulation 2 pound coin have been issued ?and in it I learnt something I didn't know before:
Aren't both the £1 and £2 sizes approximately based on the sovereign and double-sovereign? Certainly the £2 coin is, to the point that commemorative £2 coin designs have been used in sovereign sets for the double-sovereign. Therefore I think the size is fine. As for a £5 circulation coin - no, no, no (and yes, I am quoting Thatcher - of all people - there). Hopefully the polymer notes have put paid to that idea for a very long time.
I knew that the round pound's size was based on the size of the sovereign. I didn't know that the size of the 2 pound coin was based on that of the double sovereign. I am not keen on this habit of looking backwards, but when the modern 2 pound coin came into being, it was simply a plain nickel-brass collector coin, so perhaps it was thought that this connection would make it more attractive to collectors. When an actual circulation 2 pound coin was issued, it was perhaps natural to look at the collector coin and use some of its characteristics.
The bimetallic content was probably added to make the circulation coin more secure. It also coincidentally makes the coin more attractive, especially to collectors. However, at 28.4 mm it was considerably larger than the round pound (22.5 mm). Usually, the Royal Mint considers that a minimum gap of 3 mm is necessary between coins of similar type. (The round pound was not bimetallic, but both coins were round, milled, and contained brass, and in terms of denomination, they were clearly related to each other). A gap of almost double that size was clearly unnecessary.
Given the commitment to a reasonably light coinage, I think it should have been smaller. The size in itself it not a problem, though I think 12 g is probably heavier than the heaviest coin ideally should be, by 4 or 5 grams. It is, however, the largest coin I would want to see in a modern system. That leaves a problem - what about leaving room for a 5 pound coin? It doesn't. You may say, as Alan does, that we have a polymer five pound note now, and that should last us for some years. However, we cannot predict the future and cannot rule anything out. If we have a return of higher inflation, it won't be long before a 5 pound coin is upon us.
The other factors include the surely imminent demise of the 1p and 2p coin. The Isle of Man and Gibraltar already seem to be preparing for that situation. Then there will probably be a new reign by the end of the 2020s, or perhaps earlier. It might then be decided that a total redesign should be given to the coinage structure. The 5p and 10p might become bronze. The 10p could even be reduced in size slightly, along with the 2 pound coin, to make way for the insertion of a 5 pound coin.
Alternatively, electronic money could make such strides that coins become history. Then again, QE may come back to haunt us and ultimately cause unintended consequences, such as hyperinflation, leading to economic collapse. Diminishing returns, scarcer and more expensive resources, along with excess pollution, environmental degradation and climate change, might lead to an increase in disorder in the world and end economic growth and hence capitalism. Many civilisations have collapsed throughout human history. After the Roman Empire collapsed, apparently the result was a Europe where there was no economic growth or general rise in living standards between the years 500 and 1200. Of course, if collapse comes, discussing the form that five pound coins should take will be least of our worries.