If something's worth doing, it's worth doing properly. You don't actually need much to separate the codeine and paracetamol in standard 500x8 co-codamol pills (available cheaply over the counter in most UK pharmacies). 32 such tablets (the largest pack size available without prescription; the idea being that someone intent on overdosing on OTC meds might just rediscover the will to live somewhere between pharmacies ..... or at least decide it's not worth the bother, and go home instead?) providing 256mg. of codeine phosphate hemihydrate, if extracted properly. A 200 ml.bottle of codeine linctus containing 15mg.per 5ml. will provide 600mg. of codeine, roughly equivalent to 2.34 packs of co-codamol -- plus 160 grams of sucrose (i.e., ordinary coofee sugar) -- yes, you read that right; it's about 32 teaspoons by my own measurements -- and about 3.6ml.of alcohol (equivalent to about 200ml. of beer, not much to worry about for most people). I really don't want to think about consuming 160g. of sucrose holus bolus like that. Maybe you just haven't got enough sucrase in your system to decompose it all to glucose, and will shit much of it out unaltered. Where's Limpet Chicken? If anyone knows, he's bound to .....
Whether or not the fizzy lemonade actually helps with the absorption of the drug (one vaguely plausible explanation: maybe the CO2 makes you burp, you breath some of the drug-laden stomach gas back in and it absorbed into your system better that way, on account of the molecules being spaced further apart and so exposing the maximum surface area; compare how iron filings undergo chemical reactions much more quickly than a big lump of steel weighing the same, all other conditions unaltered?) seems to be ripe for experimentation.
Remember also: Codeine addiction is no easier to get out of than any other opioid addiction. The Powers That Be just reckon a 64mg./day codeine habit (i.e., the maximum you could get from 4 doses/day * 2 tablets/dose * 8mg./tablet) is breakable without too bad withdrawal symptoms, is all.