Dramatic_Hammer-
I'd have to see if a water bar could be fitted. You need to cut a groove in the doorstep and put a galvanised water bar in, there are plastic water bars but having lived with both types I'd tend to go for the metal one. That also requires that the front door has some material taken off the front part so that there is a lip which butts up against the bar.
Creating a 6mm groove in an installed doorstep (cill) is not a simple process but a small mortice chisel and some patience/careful marking it should be easy enough to achieve. You could set up a combination of router and hand finishing to achieve it but that's personal taste.
An easier alternative that might help stop wind blown rain is a large bit of moulding on the front door with a drip edge. Easier to achieve with simple skills but needs painting and I'm not sure it would solve the problem without seeing what's causing the water to get blown underneath the door.
Pro tip, don't order galvanised bars on-line. Most general builders merchants have them for less than the postage price, even if you have to walk/cycle there and pick it up.
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Airhead
I get weatherstripping from a few places but Mighton seem to have a good selection.
https://www.mightonproducts.com/products/sash-window-hardware/weatherstripping-beads/weatherseals
You need to measure the groove and the gaps obviously but it should be pretty obvious what the options are once you've measured. If possible I like to use a seal that has some acoustic properties as well as draught sealing.
Just to add, I've never found one in small quantities from the big box depots that lasts. I usually buy a quantity of the expensive stuff and then I'm fitting it to different places for a few years. My go to for that purpose has been the corner type which wouldn't work because you have the groove.