The objective is to clamp the work to the table of the machine.
As long as the clamping force is sufficient, it matters not whether the load is applied by a Whitworth, BSF, Unified or Metric thread.
Using the largest possible thread maximises the strength and power of the clamping set up.
If the worst come to the worst, you could make (After all, you do have a Mill ) your own hybrid T nuts with Imperial external dimensions and Metric threads. There are precedents fot his sort of thing.
MANY years ago, Morris cars used Whit form threads, but the engines supplied by Hotchkiss were Metric!
Howard