Replacing a complete sill isn't a difficult or expensive job. An MOT patch for another couple of years is how most car welders learn to do the more difficult jobs.
So my advice, which I've given before, is to stop worrying about the outer sills and arches and start digging at where they tie into the rest of the structure. Places like the donut mounts, diff mounts, front chassis rails especially at the joints and subframe mounts. Rot in the windscreen surround should also be considered. Use an old flat screwdriver for this work, and don't be gentle with it.
Before you go any further, remove the plastic trim inside the wheel arch at the end of the sill. Don't expect to find much metal behind it. That area is an instant MOT fail due to its proximity to the subframe and seatbelt mounts.
I'm expecting problems in any one of those areas to kill my car in the next to or three years. I can fix them,but don't consider it to be worth the time and effort. Paying someone else to do it Would buy a nice non-Omega replacement.