I was saddened to hear that adders have been killed on your property through such a misunderstood fear of this animal.
I grew up in Africa where venomous snakes are responsible for a number of deaths all the time. Normally treading on them on footpaths while they warm up first thing in the morning. Trust me they don't go out to make intentional contact with humans and large animals. They have no purpose in doing so. Rather than attack the problem by killing the reptiles, people managed their land. Snakes had little reason to be too close to human and animal habitation so plants like bamboo was kept away from houses and paddocks. Livestock feed (and straw) was properly stored so large populations of rodents didnt become pests and as a secondary issue, the rats and mice didnt attract snakes.
The last person to die on the British Isles was a small, sickly 6 year old girl about thirty years ago so I am told. I understand small dogs have succumbed to the bites, but I am amazed that such a mild venom could kill something as large as a foal. I would be asking your vet for a thorough autopsy so you can make some rational decisions on how to manage the problem. You may be surprised by the findings.
About 100 people a year are bitten in the UK and I was one of the (rather embarrassing) statistics 2 years ago. My snake handling skills were very rusty, and the snake I was rescuing from a busy road to put back into the heath was quicker on the draw than Annie Oakley. I suffered very mild effects: Small blood blisters around my nose, lips, fingers and toes. My fault, however the snake made it back to the safety of the heather. Not once did I contemplate killing it. There was no anti-venom at the A&E I visited. Maybe not a bad thing as this is often what properly finishes off a serious bite victim thanks to inexperienced medical staff. And let's be honest, you aren't going to find many doctors with oodles of snake bite experience in the UK.
I hope you can carry on living on your land with all its inhabitants, domestic and wild, with some sort of reasonable cohesion. My guess is they could all share a bit of your obvious love for animals.