Can vets put dogs to sleep without the owners permission in the UK?? Sorry if it’s a sensitive topic i didn’t know where to ask

I’m hoping no one i know finds this so badly because what the hell was this day!

So we have an electric back gate (a disabled person lives in the house) this is a new gate we got it in the summer so we haven’t had it in such rainy weather until now.

The three dogs were let out and then my step dad came back inside (the gate was shut) and then when my mum went to let them back in the gate was open and the dogs were gone they had managed to get to the main road but one came running back, the other two went further, and then one of the two came back scared.

Someone had hit one of the dogs (witnesses say they both were hit but he’s much much bigger and seems fine just shaken up) and then drove away (i do not blame this person they ran into the road but i still think it was wrong to drive off..), a lady picked up the dog that was hit and got him to a vets, she said he was completely fine the whole drive to the vets, by the time people got in contact to say what had happened, the vets had scanned him for a microchip (HE HAS A MICROCHIP) and couldn’t find it whilst i know microchips can move i just don’t understand why they would put him to sleep if the lady had said he was fine the whole drive i understand circumstances can change but he has a microchip.. can vets put dogs down if they can’t find a microchip in the UK? Surely they are supposed to check multiple times in multiple places??

Also, the lady that took him to the vets has the same gate as us and says this happens to them during rain also.. we live in the uk why would the gate not be able to work correctly in THE RAIN?!

I was not home btw please don’t attack me i’m already devastated this happened today, i don’t blame anyone in this situation apart from that stupid gate which i hate so much. I only want to know if what the vet did was legal i’m extremely confused, i will update with anything else if i find out more. Thank You.