If you've had the skirtings replaced then that's out, I'm afraid. When you pin skirtings on the skirtings are often actually fixed with a grip adhesive - all the pins do is hold the skirting up whilst the glue sets (really!) - so it matters little where you put the pins
So, I think I'd start at a corner then set out 16in centres to the approximate area you want your TV to go, then using a drill or an awl probe to see if you can hit a stud. This might take several attempts . Not what you want to hear, I know
There is, however, another way. It is possible to fix a pattress to a lath and plaster wall with oval nails (1-1/2 to 3in ones). Ideally the pattress should be either softwood or something like 9mm plywood, the nails should be driven at an angle downwards through the pattress and into the wall and the nails need to be orientated in the same direction as the laths (horizontal). This is actually nailing to the laths, and you will need to use quite a few nails, and it will need to be as large as you can get it behind the TV, but providing it is at least 20in (500mm) wide you will hold quite a bit of weight and with any luck you'll hit a stud. I've used the same sort of technique to hand tapestries in shops (off a 4 x 1 softwood plank) so I know it works. It's whether or not you are happy with disguising it