(that and the penecillin I found in the drugs drawer
This is not good. If it's not enough antibiotic to kill the virus all you are doing is helping the virus become more resistant to that particular antibiotic. Go to a doctor and get a full course and take it to completion or don't use any at all.
Actually, antibiotics affect bacteria directly, not viruses. Antibiotics attack different parts of the bacterium to either kill them directly (e.g. attacking the cell wall), or cause them to stop reproducing (e.g. halting protein or folate synthesis or DNA replication) so that the body's immune system can have a chance to fight them off. If anything, the doctor may give an anti
viral drug to take, which essentially aim to prevent viral propagation by stopping any single particular aspect of a virus particle's replication cycle depending on the drug and the type of virus. (And yes, they can develop drug resistance too.

)
Throat infections in the community are in the majority of cases caused by viruses. If you use an antibiotic in this case, at best the antibiotic will get rid of any secondary bacterial infection that might occur if the viral illness cripples you so completely that you become susceptible to bacterial infection - but this is a situation that isn't common in the general UK population outside of hospitals.
At worst (and most likely) it'll select out other bacteria that are naturally resistant to the antibiotic - in this case, likely to be certain strains of the otherwise sensitive
Streptococcus species living peacefully in the throat - and that might make the throat ailment worse by converting a viral infection (which is already damaging the affected throat mucosa) into a secondary bacterial infection that no amount of that antibiotic will help treat. On top of that are the side effects of the drug, which can make you feel just as lousy, and can be avoided if you don't need that drug in the first place.
Bottom line: if symptoms persist, consult a medical professional.
Oh yes, and
MetalPants - get well soon.
