He needs blood tests before invasive searches unless the vet can feel something suspicious and obvious by palpitation ? Insurance would not have cost more than usual, he just would not have been covered for pre-determined conditions but it`s too late now as he is under the vet for whatever is going on now, [ that`s no help to you now but might be to others who think an existing condition would cause a problem getting reasonably priced insurance ].
Blue Cross etc help people who are on other benefits, not just jobseekers so if you are on any at all, or anyone in your household is who can take him in their name, they might still be able to help, other than that a decent vet will accept a payment plan so do ask around.

Given the urgency, if there is no way to raise the money in advance, [ if you can`t find a decent understanding vet ], then I would frankly have someone who is eligible for aid take him under their name then do the honourable thing and set up a donation payment plan through a standing order until the cost of treatment is covered afterward.

[ PS, a friends large dog recently had an endoscopy under GA for an intestinal problem and it cost about £150 so do ask your vet for a breakdown of the quote as it`s a ridiculous range of figures that you have apparently been given....]