Best place to report neglected horses?

[ISeeTheLight]

On my daily drive to work are 2 small fields on opposite sides of the road. One has a horse tethered to a pole (no fence), with no shelter. No trees, hedges or buildings. Horse itself looks in ok condition.
The field on the other side is fenced and has about 6-7 horses, that side has a tree on so small amount of shade (not enough for all horses though), and I noticed today one horse in particular looks very skinny - ribs showing etc. Definitely underweight.
Where is the best place to report this to? Horses are outside in all weather, it's pretty hot here today with not a cloud in the sky. I'm in the North East.
First thought was RSPCA but I don't know if they'll actually do anything from experience with cats.

Despite the majoty vote here on rspca being crap with horses I have had 2 good experiences in recent years. Give them a ring. Take pics if you can so you can monitor and report again if necessary.
I supplied pics once of a horse repeatedly tangled and it was removed.

World horse welfare are best bet. However horses do live outside, and if one horse is skinny but others are ok it may be elderly.

I don't like to see horses tethered but it's not illegal so if in ok condition, has food and water there's not much to be done.

Ring the BHS. They can advise the best charity to contact.