How much luck are you having with them? even in the UK they will probably need stacking in order to get the shutter speed down to the desired amount of x2 framerate - Adam
I don't think that will be necessary. For me (in Denmark) for very bright days the ND8 or ND16 is sufficient.
Last week on a sunny afternoon I used the ND8-filter combined with the POL-filter (Polarizer) and in Auto the camera set itself to the desired 60 shutter speed (4K30fps), ISO100, EV=0. No tweaking needed!
I find stacking the POL-filter + an ND-filter is often very useful. The POL-filter alone cuts down the bright light about as much as an ND2 or ND4.