Dog skin allergy help

Hi. My daughter took on a bulldog that was being given away. She wanted to help and didn't know about the skin problems. She's not working at the moment and been told by the vet, it's £500 for an allergy test. She's not got it. I can't help much either. Any advice on how to get help please? I told her not to take it on but she wanted to give him a nice home, but it's skin is sore on his face. She's been putting suncream on it and put him on a raw food diet. But he isn't getting treatment as she can't afford the allergy test to find out what it is!

Sorry. Sudocream.

She can't get insurance as its an ongoing condition. The vet gave antibiotics and an injection and shampoo, but it's come back. She can't afford the bills but can't rehome him as the kids love him. Not in the right area for PDSA or other charities. RSPCA? Do they help with low cost treatment as their website doesn't mention it.  I wish I had the money to pay the vet. He's an English Bulldog