But for those of us living about latitude 42 her suggestion that we should limit our intake to 1000iu/daily without 25(OH)D testing is dangerously misguided.
There is absolutely no risk to consuming less than 10,000iu/daily therefore to imply there may be is totally unjustified.

This paper Diagnosis and treatment of vitamin D deficiency provides a useful up to date summary of current knowledge.

We need to understand that each 400iu/daily of D3 will raise 25(OH)D by between 7-12nmol/l. so 1000iu/daily will raise status by an average of 9ng/ml = 22.5nmol/l
the average UK adult has a 25(OH)D status of 40nmol/l at the moment raising that to 66.6nmol/l leaves them insufficient.
At the end of summer (or what we call summer in the UK) the average UK adult will have a status of 70nmol/l raising that to 92.5nmol/l still doesn't enable them to store D3 for the winter.

Look at the plots here of stored cholecalciferol (D3) showing what happens when people get ample sunshine or adequate supplementation. We need to aim for a status of at least 50ng/ml 125nmol/l and preferably 60ng/ml to achieve optimal health.

The average UK adult will not and cannot achieve that on 1000iu/daily.