The general advice when bringing medication is

• bring it in the original packaging. In the case of the prescription this includes the sticker with your name, doctor's name, date etc. Never just bring two or three loose pills in some other container
• if you have a copy of the prescription, or your receipt with your name, bring that too
• if the item doesn't require a prescription where you live, but is controlled where you're going, bring a letter from your doctor (something I did recently bringing Sudafed to New Zealand)
• declare on the landing card if they ask about medication (don't just say yes to the question, circle the word medicine in the question, see https://transpacifictrip.wordpress.com/2015/07/20/landing-card-tip/ )and be prepared to answer questions about your landing card

My experience coming into a variety of countries (but not the UK) with prescription drugs is that they ask "what medication do you have?" and I say something very short, eg "antimalarials for Vanuatu" and that's the end of the conversation and nothing happens. But you need the paperwork etc to back you up in case they want more details.

Since you have no paperwork and probably don't have the right container, don't make things worse by not declaring. If the landing card asks about medications, say yes. Be prepared to tell someone what the medication is and what it's for (eg "an antibiotic" or "for my asthma" or whatever it is.) It's most likely that no-one will care, but if you run into someone who does, the fact you were open about having it is more likely to get you through without your meds confiscated.