First Kevin J , I wish to apologize for giving some bad information. The Iron Dukes and the Tiger
were allowed to be kept under the Washington treaty.
The Thunderer, King George V, Ajax, Centurion were also kept but only until the completion of the Nelson and Rodney. Likewise the US kept the North Dakota and the Delaware until two ships of the West Virginia (Colorado) class were completed.
Other Navies were allowed to keep some older and much less effective ships, Like Italy was allowed to keep the four Regina Elena class ships
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but chose not to fairly quickly. With 2-12in and 12-8in guns their ability to sand up to a dreadnought was not good.
Both Italy and France were allowed to
lay down new ships in 1927. 1929 and 1931 but only up to the total tonnage and as replacements for scrapped ships. (it would take 2 or 3 of these old clunkers to equal one new battleship)
It appears (to my eyes anyway) that the treaty was effective in it's first years. However most of countries were either recovering from WW I or natural disaster (Japan's earthquake).
The treaty had the unintended consequence of starting the the heavy cruiser race. Numbers and total tonnage of cruisers was NOT governed by treaty, only the cap of 10,000tons per ship and a max caliber of 8in for the guns so this race for
2nd class battleships was on. No major country built cruisers of less than 10,000tons and with less than 8in guns for a number of years. Britain built 13 of the county type cruisers, all laid down within 2-3 years, for example and while not as expensive as a battleship they were not cheap, one cost is give as £2,180,000 mean. Which would pay for rather extensive modifications to an older battleship.
This cruiser race caused a meeting in 1927 which failed to reach agreement but did result in the 1930 London treaty, which did impose tonnage restrictions on cruisers, destroyers and submarines.
Unfortunately the Iron Dukes and the Tiger needed some serious work to stay effective even in the late 20s when they were taken out of service (Tiger excepted). Since they burned coal with an oil fuel supplement they would need new boilers to stay current, both anti-torpedo and anti-bomb/longe range gun fire protection was deficient. The guns were fine but the mounts limited the range. The fire control was falling behind. Some of these objections might not count for much if you plan to send them to some outpost and anchor them in place but short ranged "shore battery" with 2nd (or 3rd) class fire control might not be a good plan.
The Iron Dukes and the Tiger don't run afoul of the treaties until the British start laying down the KGVs but with the treaty restrictions on carrier tonnage you can't just cancel a few KGVs and build more carriers without breaking the treaty (which was crumbling pretty quick in the mid to late 30s anyway). The Americans and the British sticking to it better than the other navies but since they were most powerful navies under the treaties they had the least reason to cheat.