So you're implying that the turtles went further than the Copdock/Washbrook junction all the way to the Copdock Mill interchange at Ipswich, but were then taken down?That didn't happen. There was another reason why that section of the A12 went from being lit to unlit. This is my recollection of events (which may be a little woolly, as I was very young at the time!)
Before the early 1980s:The A12 used to run straight into Ipswich town centre, and may have been lit all the way from the town centre down into Copdock. The A12 is such an old road (and quite possibly still on the Roman-built alignment at this section), and over the years it had attracted a lot of ribbon development with associated driveways, as well as side turnings, bridleways and farm tracks. Because of the amount of hazards, lighting of this road all the way down from Ipswich and through Copdock would have been justified.
Early 1980s:Then in the early 1980s when the A45 Ipswich Southern Bypass was built (probably 1982, as that's when Wikipedia claims the Orwell Bridge was opened), an opportunity arose to take the A12 away from Copdock village on to a completely fresh piece of road, presumably as a safety measure to avoid the existing ribbon developments, side turnings, bridleways and farm tracks etc. With the A12 now having practically no obstacles between Capel St. Mary and Ipswich, there was no need for the road to be lit. Hence the A12 went from being lit to unlit in this section, but only because the A12 had been diverted onto a fresh, hazard-free section of dual carriageway. The lighting through Copdock remained on the declassified road, and the twin Fabrikat-style lighting columns with the uptilt brackets still exist to this day,
but now have SON lanterns on (I cannot identify them, but I know that sotonsteve can!)
Very late 1980s / very early 1990s:The GEC turtles at Capel St. Mary (junctions 32a and 32b) appeared in the very late 1980s or possibly very early 1990s, a few years before the SGS204s appeared at Rivenhall End (junction 22 to junction 23) and the Mountnessing Interchange* (junction 12). I would even suggest that had they appeared at the same time, they would also have been SGS204s or '203s. The turtles were installed as part of safety works along the A12 at Capel St. Mary.
As previously with Copdock, the A12 at Capel St. Mary also suffered from ribbon development, side turnings, bridleways and farm tracks, but instead of an off-line diversion, these various turn-offs were 'tidied up' into
parallel-running roads and fewer but longer slip roads on to and off the main carriageway, with new lighting thrown in as part of the scheme.
Very early 1990s/early 1990s:Needless to add, the Capel St. Mary turtles were installed too high (at 12 metres) for a two-lane dual carriageway, and when they were replaced with Thorn Alpha 2000s after an unfeasibly short period of time, the opportunity was also taken to replace the top sections of the Petitjean-style columns (which add two metres to the overall mounting height) with special brackets which gained practically no height but still had a two-metre outreach, hence the whole main carriageway installation was reduced from 12 metres to 10 metres without uprooting the columns!
Also worth noting is that the columns at Rivenhall End and Mountnessing Interchange were very over-engineered, with 15m octagonal columns combined with top sections that gain no mounting height. As previously stated, when the opportunity arises to replace them (Brentwood Bypass reconstruction, Witham Bypass reconstruction and the Kelvedon South safety scheme)
the height is brought down to 12 metres with new columns, but where this hasn't been done, the policy appears to be
to replace knockdowns with lower-height columns, presumably in the belief that within a few decades, most of the installation would have reduced from 15 metres to 12 metres thanks to the A12's appalling safety record!
* Not the Trueloves interchange as previously stated, which is one junction up (junction 13). Although this junction is also lit, the main carriageway is not.