Bloody road rash. I'd take it to a good local smart repairer.
clay it first... you may find a lot of it is tar.
As it's under the arch I think you could get away with being a shade out.
Have a look at the chipex website. I used their stuff on an Alfa Berra that I had with decent results on road rash.
And who here has had things like this cut out?, I'm guessing none, apart from it being a subframe , don't scaremonger please. Rotary wire brush the area back and have it treated , job done.That looks like quite deep rust and if you are planning to keep the car any length of time it will be worth doing it correctly before it spreads.
So with limited knowledge I think the only way to get rid of rust is to cut it out, the fact it’s on a seam if less than helpful.
I'd scrap the car TBH. Or, sell it to me really cheap.
They obviously heard its a rust bucketWBAC just have me a price £15k, it’s yours for £15k + £1 no haggling
15k. I would jump at that before it dissolves on the drive. Not far off by the looks of it.