Just buy made up cables and bury them in the wall. Scart leads are a pig to solder, and I have never seen rewirable hdmi plugs. Even if they were available, they would be minute and require precision soldering.
As it's a stud wall, I use a 1G plasterboard back box, with the back cut out as way to bring all the AV cables in and out. A blanking plate over the top with a cut out sawn and filed to allow all the cables to snugly exit.
Make sure when framing up the stud wall that they do not put any horizontal noggins in the section of the wall between the sky box and the tv. Then at any future date you can pull more cables if needed.
Install a large area or horizontal timbers at high level to ensure a good fixing for the tv's bracket (but remembering to leave space for your mains socket and your 1G "av box" somewhere where they won't be seen behind the tv.
If you want a neater job, CPC do some front plates for AV connectors.
