As a building contractor we have endless problems with skips,
It is illegal to add or take anything from a skip without permission,
but if someone has fly tipped you, and the items are in or on your property(in your skip) you are now responsible for them, if you take them out of the skip and put them on the road, you are then fly tipping.
we tend to use a license collection service, the local council will supply you with the contacts, they cost less than skips these days, but you do not have to pay for others rubbish,
You can appeal to the council to come and pick up the rubbish, but you will be very lucky if they do,
fly tipping is a huge problem as recycling yards are privatised and make it difficult for households to dump easily, so they dump their ripped out bathroom tile and plasterboard into farmers gateways,
as plasterboard cannot go into landfill because of the paper on it,
I would get advice as if you do put the items on the ground you could end up with a hefty fine.