A few things to note:
1. The WLTP quoted range of 140 - 144 miles is not real world figures. Mini say the following on the website “Mini Electric figures are for comparison purposes and may not reflect real life driving results which depend on a number of factors including the starting charge of the battery, accessories fitted (post registration), variations in weather, driving styles and vehicle load.”
2. In the Winter electric cars have a significantly reduced range, this is due to how batteries work at low temperatures and the extra demand placed on them due to needing more heating, lights, etc.
3. The way you drive, regardless of the driving mode selected will also make a difference. Do you try drive in a careful way or use it like a go cart? Do you use the brakes at all or do you anticipate well and use the regen brakes most of the time?
As to the range I am getting: At the moment on cold days I am seeing a quoted range of around 100 miles. The actual range achieved can be more than the range shown as the figure tends to be pessimistic.
In the summer when I first got the car I was seeing a quoted range of 120 miles. I did make a trip to Blackpool and back which is 112 miles round trip, when I got home the car was showing a remaining range of approx 30 miles, so I have seen the full quoted range is possible. I did the trip in Green+ mode, did not go over 50mph on the motorway and was very careful to drive as economically as possible.
If you had done a bit of research before you bought the car then you would also have seen that the real world range of the Mini was quoted as 115 miles. As with all electric cars the WLTP is not really possible and real world ranges should be looked at. You can see this at the following site:
All details and specs of the Mini Electric (2021). Compare price, lease, real-world range and consumption of every electric vehicle.
ev-database.uk
If you also read / watch any reviews of the Electric Mini which I presume you would have done before you bought the car, they all say that the range of 144 miles will not be possible in the real world and to expect more like 100 miles.
I don’t think you have faulty cars, they are performing the way all electric Minis seem to be doing, in line with other electric cars, i.e. giving a less than WLTP range in the real world. I don’t think you will get anywhere with BMW as they make is clear the 144 miles range is for comparison purposes only and not a guaranteed figure.