Blasto? That's a hard question to answer with the information given. But, I'll run the rapids.
You say that the dog might have been exposed 12 months ago, and then later suggest that the dog has been "affected" with this breathing problem for 12 months. (emphasis mine for reasons below)
It's true that blasto can lay dormant or sub clinical for many months before becoming acute. So, a dog may or potentially could be exposed many months earlier before an acute sickness develops.
However, I doubt that any dog would have an acute blasto infection for 12 months without any form of antifungal treatment. That is to say, I think, that an acute blasto infection would kill any dog in short order without treatment. 12 months of active respiratory distress likely rules out blasto.
Your geographical location probably rules out blasto as a culprit. Blasto is fairly territorial. I can't say for sure, but I don't think that anywhere you might be paying in Pounds and blasto's home range overlap. Feel free to correct me. But I didn't think that blasto was outside of North America.
Lastly, I think that most of the board here would suggest that further testing is needed. If you do suspect blasto, or another fungal critter like blasto, then fungal titers and the much loved Mira Vista antigen test are in order. I'm betting that your vet is planning on testing for much more than just a fungal infection. The MV antigen test, and for that matter a titer, shouldn't set you back a fraction of that 300 pounds.
Several caveats to consider when reading the above;
1) I'm not a vet.
2) With blasto, just about anything is possible. As soon as you think you know just what blasto can do, it does something unpredictable.
3) There are several other fungal infections, other than blasto, that might be endemic to your area.
4) I have no idea the costs of Vet services in the UK.
5) Internet diagnosis of complex issues like Blasto is little more than fortune telling. If you're comfortable with your vet, take their advice.
6) People like us (blasto survivors at best) are on a hair trigger to see blasto in every cough. And that's a good thing. Blasto has a habit of being hard to get rid of, so we're always on the watch for the infection.
I know, that probably doesn't help too much.
Kash