I think astigmatism can cause something like the halo or spikes that have been described. For instance I believe it can turn points of light into lines. It's corrected by cylindrical lenses, notated in the "CYL" column of the prescription. This is added in the prescription part way through the eye test when you're shown the collection of black dots on a white background, and the optometrist asks, 'are the dots clearer with lens number one... or two'. With one of the lenses the dots might look clearer and less 'smeared'.
To the OP, I reckon it's normal to need a different lens (or perhaps no lens) when viewing things closer to you, if you're short-sighted. Rarely, if ever. do short-sighted people actually need their full power distance vision lenses to read a book. They might need a lens but it would ideally be less powerful, typically by a dioptre or two. By the way, this is usually checked at the end of the eye test when you're given a 'book' to read (a book which might in fact be a piece of card) and asked which is the smallest sized text you can read. Then the optometrist holds a hand lens over your trial lenses and says, 'what's the smallest you can read now?' The hand lens is plus 1 dioptre, effectively weakening the prescription. If you express a preference for this, the resulting weakened lens for close-up vision will become a 'reading addition' on your prescription, which might be notated as "ADD".
Those with a mild refractive error (i.e. those who are only slightly short-sighted) may be able to read a book without a lens (i.e. reading without glasses) because in these circumstances the natural lens in the eye is made to work in order to focus the eye. Those who have gone through life able to focus on a book with no lens have been lucky! I could until a few years ago.
If you have very mild myopia (short-sightedness) which needs correcting only in the worst of conditions (e.g. distance vision in low light when your safety is at risk), the power of your reading addition might in fact be larger than the power of your driving glasses, shifting your reading prescription from negative numbers into positive numbers (in the SPH column of the print out they give you). Where this is the case, your distance vision would be corrected by a lens with a negative sphere shape (a lens thicker at its edges than its centre), whilst your near vision would be corrected by a positive sphere (a lens thickest in its centre). Looking into the middle distance in good light might need no specs at all. And those who don't need specs all the time are the even luckier ones!
This last scenario may be what's happening in your case. (Feel free to share your prescription here if you like. Specsavers might have given you a printout of the numbers for SPH, CYL and ADD.)
In either scenario, different specs for driving and reading. If the difference between your near lens and your distance lens is huge, your optician might recommend a set of specs with varifocal lenses.
Anyway nowt wrong with your eyes, and your optician is doing a good job.