How easy is it to put a tv aeriel socket in the wall and wire it up from one main aeriel in the loft?

You certainly don't need an electrician as there is no mains voltage involved. The big DIY stores, TLC, Maplins and Screwfix do kits. You need a splitter in the loft plus the connectors to suit. A length of coaxial (TV) cable and an outlet. A little surface mounted outlet is neat and easy to fix. The cable routing is a matter of personal choice, but as the other person said poke a neat hole in the corner of the bedroom ceiling push the cable up into the loft, and run it down to and along the skirting. If you want to be neat you can buy white self adhesive mini trunking which you stick to the wall first, pop the cable in and press fit the cover. Look on the net for advice on fitting connectors to coax, it can be puzzling if you haven't done it before. Keeping the braid separate from the centre wire is paramount.