I've searched and found answers around this but none hitting it squarely. Usually I travel with a zippered clear 3-1-1 bag (Sephora travel bag). It's a non-issue because I've had TSA pre-check for years and use a 2nd bag for medical liquids (contact lens fluid, eye drops, etc.). My understanding is LHR doesn't allow the medicines bag, so I can work with that, but I'm also seeing that the zippered bag I use will not be allowed and I'll be asked to switch everything over to a 20 cm x 20 cm press-to-seal bag at security. My question: If I start off in the US with a 20 cm x 20 cm bag, will that satisfy security at LHR and DUS on the return flight? Or will coming off a US flight mean switching the contents into their provided bag, even if it's the same size?