There's very little available on Freesat (or by accessing the Free-to-Air channels one can get using a generic satellite box) that's not available on Freeview so, if you've already got Freeview, you might be wasting your time.
Here's a full list of all of the free channels carried on the Astra satellites that transmit both Sky and Freesat:
https://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_Astra_28.2%C2%B0E_(Ireland_and_the_United_Kingdom)
If you still want to go ahead though, a decent aerial installer will be able to assess the situation and advise as the whether it's best to mount a dish on your chimney or atop a wall-mounted pole:
https://www.digitec-aerials.co.uk/uploads/1/3/2/6/13260184/satellite-dish-on-a-pole-or-mast_orig.jpg
Note that I referred to a
decent aerial installer in my previous paragraph. They're very much in a minority, with cowboy firms getting most of the business. Don't even consider any firm that doesn't hold CAI membership. (Use the search facility here:
https://www.getmeviewing.org.uk )