No point looking at the sky box signal meter. It's useless, IMHO.
We have a signal at a frequency 10GHz from satellite to LNB. As has already been pointed out, that
will suffer significant absorption from rain and snow. These are laws of physics, which even Murdoch can't corrupt.

Whether you see any difference to the picture quality depends on whether the signal is still sufficiently above the noise level when it gets to your receiver, even after some absorption.
The main factors in this are as follows:
1) Obstructions between dish and satellite
Not much you can do about this other than limited movement of the dish, but bear in mind that if you have trees in the path they absorb the signal to a degree, and when trees get wet or have snow on them they are almost impenetrable to a signal at the frequency we are talking about.
2) Size and quality of dish
The larger the dish the more "gain" it will have, and therefore the stronger the signal collected and received by the LNB in the focus of the dish. If it's rusty, bent out of shape or generally knackered it will be less effective. A larger dish will help if you are having problems.
3) Alignment of dish and LNB
The dish has a very narrow field of view when it comes to receiving signals so must be precisely aligned when installed. Years of birds perching on it, the wind battering it and the supports rusting will have affected that. In addition, the LNB at the focus of the dish must be angled so that its' orientation matches the polarisation of the antennas on the satellite. This is referred to as "skew".
4) LNB Quality
Since the LNB performs the first stage in the receiver chain, it is the part that determines the sensitivity of the receiver in general. The set top box is fed a much amplified signal at a much lower frequency from the LNB so its receiver is much less critical. If the LNB has been there for years, it is quite likely that more modern devices have much better performance, and that the original performance of the old LNB will also have deteriorated after years out in the elements.
So, these are the areas that need addressing. No reason why you can't improve the above yourself. Get a signal strength meter from Maplins, etc. and give the dish a tweak. If it looks knackered, get a new one. Ditto the LNB. There are plenty of guides on the internet to tell you what to do. No point in moaning about Sky. If you can't get a decent signal it's only going to be the above factors, which are all under your control (although, as said, if you have trees in an awkward location you might be stuck with what you've got).