The most sustainable alternative must be development of the main urban area, since that provides access to the necessary infrastructure. So I think Option 2a would be best.
Second best would be combining lower development at Bedford with development at Colworth or Twinwoods. The latter would make use of brownfield land, at least in part, and it would support Bedford town centre. The main objection concerns the A6, but this is a longstanding problem, which needs in any case to be resolved to cope with extra housing already planned plus access to the East-West railway station in Bedford: infrastructure funds from development at Colworth or Twin Woods could be used for this.
A third possibility might be lower development at Bedford plus a new settlement at Little Barford, but this would need to be considered in a longer timeframe and in conjunction with other planned new development in the St Neots area. I assume that the new East-West St Neots station would be East of the Great Ouse and linked to the East Coast mainline. Depending on the exact location of this station, development at Little Barford could provide dwellings within walking distance of the station. Access to the existing St Neots Station and to the town centre and other facilities of St Neots would be relatively straightforward. But such development should not be considered in isolation and would need to take proper account of the other development planned in the St Neots area and of the pressure on facilities there.
The worst option (eg 2c) would be to create a new settlement West of the A1, eg at Dennybrook. This has been recognised for many years as an area of tranquil beauty and rural peace. Turning it into a built-up suburb of St Neots would run directly counter to the Borough Council's own Vision for the future. The whole character of this area would be changed. Heritage buildings would be engulfed in new development. Local residents, who have come here for the peace and quiet, are strongly opposed.
This area has highly productive farmland growing food which the UK would otherwise have to import. It has wide views and big skies. A new settlement would bring light pollution and threaten the wildlife of the area. In my own garden I have recorded over 100 species of birds, as well as Great Crested Newts, the caterpillars of the Small Eggar moth, Marbled White butterflies, Norfolk Hawker and Scarce Chaser dragonflies and other rare species of invertebrates. These depend on the surrounding countryside for their existence. The narrow lanes of this area contain rare plants such as bath asparagus and crested cow-wheat. All this would be put at risk.
Because this area is west of the A1 and the Great Ouse, it is physically separated from the town, the existing station and the proposed new station of St Neots, and there are inevitable traffic bottlenecks for anyone wishing to travel eastwards. If the population was substantially increased, car use would also increase, and so would congestion and carbon emissions. The Dennybrook site also poses substantial flood risks.
I simply do not understand how this could be viewed as a "sustainable" option.