yo... I dont know if you were actually prescribed any painkillers, as I never read any post by you to that effect, however I did a likkle bit of research.
Paracetamol/NSAID (brand name - Aspirin, etc.) - Not known to cause paranoia.
COX-2 selective inhibitor (derivative of NSAID - brand name - Celebrex) - Not known to cause paranoia.
Neither of the above have been known to cause any kind of mental side effects.
Narcotic [specifically Opiates] (brand name - OxyContin, OxyCodone, Morphine, etc.) - Has been known to cause anxiety and paranoia, regardless of whether the patient is "withdrawing" (from lack of the drug) or not.
I seriously doubt they're just handing out Opiates in the UK for headaches, however nothing would surprise me.
There are other meds employed for pain, such as Gabapentin, etc. however I think that it's probably unlikely you've been prescribed these.
I'm not a doctor or anything, though I have an extremely... shall we say "close" relationship with drugs (of all forms, not just the "fun" ones). I spent most of my teenage years educating myself about narcotic and non-narcotic medications.
Good luck.