Large White Patch On Platy's Back

Looking for a suggestion of what could be going on with this Platy. large white area in front of dorsal fin. no other signs of anything goign on with other fish in the tank, and no odd behavior seen as of yet. First noticed a small spot there about 2 weeks ago, it has gotten bigger since. It likes to hide in the variuos Spongebob houses I have in the tank, and the pineapple one in particular i have noticed it poking through the window hole which is barely larger than it's body, i'm thinking it could be an abrasion form squeezing through there but not sure.
 

White patches often tend to be fungal infections, caused by a previous injury, which obviously matches up with your observations.
 
SInce you're in the US, I can't suggest any appropriate medications, as I'm not sure if any that are available in the UK are also available over there.

I'm sure i can get some sort of genereal anti bacterial/fungal medication. do I need to isolate this fish or is it okay to treat the entire aquarium it is in? It's a 10 gallon (38 Litres) if that matters. other tank mates are 1 Molly, 2 octocinclus, 1 corydora and 1 golden mystery snail.

In this instance, it would be better to isolate the fish, if you are able to do so.

Thank You

My Corydoras bronze always ends up scratching her dorsal fin in the same spot. I scoop her out of the aquarium (soak the net first for 30 minutes so it doesn't scratch her) and I use a q-tip soaked in melafix and I rub the fungus with the q-tip for a few seconds (very lightly, some of the fungus will come off). Then I use another q-tip soaked in pimafix and I do the same thing. make sure to already have the q-tips soaking and ready to use so the fish is out of the water for the shortest amount of time. Stress may make it worse.
I've done this a couple times and it's worked really well. Now I just let it go away on its own. It shows up and goes away within a couple days. Yours looks like it needs intervention. If you don't want to quarantine your fish this would be a good option, maybe. 

Thanks. This fish is very skittish so dabbing it with a swab is not an option, but i appreciate it. I think my best option is going to be to get a small "hospital tank" going for it.