Start with the rules, not the packing
The most important thing to establish before anything else is whether what you take is treated differently in the UK than it is at home. Medicines available over the counter in one country are sometimes controlled substances here, and a few common preparations fall into that category. The reverse is also true, which catches people out on the return leg.
The UK government publishes guidance on bringing medicines into the country, and it distinguishes between ordinary prescription medicines and controlled drugs. If any of your medication is a controlled drug, there are additional requirements, and travelling with a supply above a certain duration can require a personal licence arranged in advance. That is not something you can sort out at the airport, so check it weeks ahead rather than days.
Three sources will answer almost every question: the official UK guidance on travelling with medicine, the UK embassy or consulate for your departure country, and your own prescriber or pharmacist. Ask all three if you are unsure. None of them charge for the answer.
The paperwork that actually helps
Original packaging with the dispensing label
Keep everything in the box or bottle it was dispensed in, with the pharmacy label showing your name and the medicine. Decanting a week of tablets into an unlabelled organiser is convenient at home and unhelpful at a border.
A copy of your prescription
A printed copy, plus a photograph on your phone as a backup. It should show the medicine name, ideally the generic name rather than only a brand, the dose and the prescriber.
A letter from your prescriber
For anything unusual, injectable, controlled, or requiring equipment such as needles or a pump, a short signed letter is worth requesting. It should state your condition in general terms, what you take, why, and that the quantity you are carrying is appropriate for the trip.
Generic names, written down
Brand names differ between countries. If you need to replace something in the UK, a pharmacist can help far more quickly with a generic name than with a brand they have never heard of.
How to pack it
Medication belongs in hand luggage, always. Hold baggage can be delayed, misrouted or held in a temperature that ruins a temperature-sensitive product. UK airport security permits liquid medicines above the usual 100ml limit when they are essential, but you should expect to declare them and be ready to show supporting documentation, and it is worth allowing extra time for that conversation rather than arriving with minutes to spare.
Split your supply if you can. Half in one bag, half in another travelling companion's bag, so a single lost item is not a crisis. Carry more than the trip requires, because delays happen and replacing a prescription in an unfamiliar health system takes time. If something needs to stay cool, use a proper insulated case rather than improvising, and never put it in the boot of a car on a warm day.
Time zones and dose timing
This is the part where people improvise and should not. Shifting the timing of a medication across time zones is a clinical question, and the right approach depends entirely on what you take. Some medicines tolerate a shifted schedule easily, others do not. Ask your prescriber or pharmacist for a written plan before you fly and follow it rather than working it out on the aircraft.
Arrival day, practically
Arrivals are the moment when a carefully organised trip tends to unravel. You are tired, the terminal is large, and the medication is at the bottom of a bag you cannot open in a queue. A few things make it easier.
Keep one small bag on your person containing medication, documents and water, separate from everything else. If you need to take something during the transfer, tell your driver before you set off rather than in traffic. Do not put the medication bag in the boot. And build slack into the day: an early check-in that is not available yet, a long queue at the border, or a delayed flight can all push a dose several hours later than planned.
If you are arriving unwell, travelling with someone who is, or carrying equipment as well as luggage, a prebooked car is worth the money purely for the certainty. A driver waiting in arrivals with a fixed price, a tracked flight and a boot big enough for everything removes the part of the journey that is hardest when you are not at your best. Rushxo transfers are fixed price with flight tracking included, and you can tell us about extra equipment, a wheelchair or a cool box when you book so the right vehicle turns up.
Save two things to your phone before you fly: a photo of every medicine box you are carrying, and the NHS 111 number for non-emergency medical help in England. Both take a minute and are useful precisely when you cannot think clearly.