You're very welcome. If it helps further, our dogs are breeds which stereotypically hate other dogs (we have two Caucasian ovcharkas and a Fila Brasiliero) and when we rescued them, all three were on death row due to apparent extreme aggression and being uncontrollable. All thee live harmoniously now and you couldn't ask for more loyal or sweet dogs.
Also, in case it helps, never once has any of them turned their aggression on us, even during the worst of their fights I could step into the middle of the fight and rub their ears, and their desire not to hurt me by accident was always stronger than their motivations for fighting, so they stopped without me ever being caught in the crossfire. Disclaimer, not a course of action I recommend but I know my dogs so can do it with them.
We're not in the UK, but where we are you can get old tyres for free from garages, and Pinterest has some great ideas about how to make dog playgrounds with them, even if you're limited for space. Pinterest has some great ideas for dog mental stimulation in general too. I have trained my dogs at shuntzhund, so when I'm walking them it gives them extra mental stimulation. Its great fun too, and easy to teach, so might help to keep a collie entertained?
When we rescued ddog1, we were told that she was a hopeless case, they were then trying to corner her to put her to sleep. She had attacked five different behaviourists (I'd seen most of it as I was at the rescue centre for other reasons), numerous staff members at the rescue centre and had killed many many other dogs. Now I trust her as much as I would any dog, she is bombproof and a joy to be with. It really does take patience and perseverance, and unless you know what you're doing the help of an expert, but situations are rarely hopeless. It is tough though, hang in there x.