The Japanese Maritime Self-Defense Force is one of the largest navies in the world, ranking at about even with the Royal Navy.
True, but as said, doctrinal-political reasons prevent it from serving in the commerce protection role.
South Korea also has a pretty large blue-water fleet as well.
Only in the sense that individual vessels can transit across oceans; the same is true of basically every navy out there, with tiny gunboats being hauled from the manufacturing country to the user country in a completely non-operational manner. The Korean vessels lack national support assets allowing actual operations in distant waters; the Royal Navy has those assets, even if in dwindling numbers, and has traditionally operated ships of longer individual reach from a global network of national bases anyway.
Both, IIRC, have participated in the anti-piracy patrol around East Africa.
True enough, but something of a special case in commerce protection, largely ineffective and symbolic, and not particularly related to the capacity to protect national commerce in an "actual" crisis. Not that classic convoy protection would really serve a purpose nowadays, regardless of the nature of the crisis; technology has moved past such things already.
How is commerce raiding countered in Star Trek? We have seen two convoy battles, in "Rules of Engagement" and "Sons and Daughters", with WWI-style tactics where the escorting vessel scrambles to meet the attackers and leaves the transports defenseless. That's very different from how convoys today would fight, with "fortress ships" keeping close to the proteges and fighting back with long-range weapons - but Star Trek doesn't seem to have that sort of weapons technology. Would the Ferengi escort merchant ships? Or would the merchant ships be their own escorts - i.e. would trade take place with those mighty
D'kora Marauders in times of crisis, or perhaps even on a regular basis? We don't really know what it takes to keep trade flowing in Trek, or even whether keeping trade flowing is a worthwhile pursuit. For the Ferengi, loss of commercial shipping might well be somebody else's problem, or indeed everybody else's problem; the Ferengi would just wait for the profits, or alternately the insurance money, as the goods themselves would matter nothing to them.
Timo Saloniemi