Seen your Wanted thread ?
Cheap is relative in this country. For new ones, they're all imported from Europe, which these days is a deviant form of entertainment in its own right
They'll be expensive enough to now attract UK VAT in the first place, then the size and shape of them makes for high transport costs too. From memory they weren't landing for under £50 per unit before Brexit, you had to buy several to combine transport costs to even get that. And I don't think those were even the diesel ones with the sound deadening layer (covered with oil on your photos).
But now I look closely, the holes for all the front and rear mounting points on your undertray look fine! Was it hanging down at one side when you removed it? If so, I'd guess the last garage to refit it only replaced the fastenings for the side mountings on the tops of the legs - due to sheer laziness, sadly. I see one of those has snapped.
It would be worth checking under the car to see if there are speed-nuts (the metal clips that provide the thread for bolts to be screwed into) in positions corresponding to all the other mounting points - or at least enough of them, at both front and back edges. I do wonder if all you actually need is a few of these and/or the bolts to go in them, then you could re-attach your existing undertray to your car.