You're really not coming across well, OP. You have 2 insured dogs as well? So 4 dogs in total? That's a massive obligation without the healthcare costs. And this pup is 8 months old, so you probably got her in the last 6 months, knowing she would have high healthcare costs. In the last 6 months then you have lost your job and are relying on benefits/selling things to fund unexpected costs - so sounds like before you lost your job, you didn't have a savings cushion. That's not a criticism, lots of us don't, but it should be a factor in taking on large ongoing financial obligations (such as a young, apparently uninsurable disabled dog) if you are in a financially precarious situation.
Honestly, it comes across that you can't properly afford the pets that you already had, so taking on another seemed like a daft idea at best. Saying you 'rescued' her doesn't make it any better I'm afraid - one single person is rarely the only option an animal has, there are many reputable rescue organisations around. And yes, it sometimes can be a reasonable decision to euthanise an animal if healthcare costs cannot be met. Euthanasia has no negative welfare impact, and welfare must be the primary considersation.
The PDSA is a charity, and £600 is an absolute bargain for the service you've just had, in private practice that would likely be tripled. In order to be able to provide their service the PDSA has strict rules on eligibility, which you haven't met.
It's a sad situation all round and I feel for you, you must be under a lot of stress and pressure with financial worries and worrying about your dog, who clearly means a lot to you. But none of this is the PDSA's fault, or the fault of posters on this thread, and your rudeness to both does not cast you in the most sympathetic light.
I hope she recovers well and you are able to find a more stable way forward for all of you.