No Separate 3-1-1 Bag for Medications at Heathrow Airport

You are permitted a single quart sized bag at TSA as well. See https://www.tsa.gov/travel/security-screening/liquids-rule. I have seen the rule enforced (more often) but also seen it ignored by security personnel.

Did you tell him/her it was medicines? Or did they just assume they were cosmetics?
Medicines are exempt from the limit, but you may have to prove it (prescription).

The TSA are irrelevant at Heathrow, it is not "3-1-1" (not sure what that is). It is EU rules that apply, and that is (from the official website: https://ec.europa.eu/transport/modes/air/security/aviation-security-policy/lags_en )

liquids in individual containers with capacity no greater than 100 milliliters packed in one transparent one-liter re-sealable plastic bag ;
liquids which are to be used during the trip for medical purposes or special dietary requirements, including baby food;

Chris, 3-1-1 is the shorthand for the TSA rule on liquids in carry-on luggage. You are allowed to bring a quart-sized bag of liquids, aerosols, gels, creams and pastes through the checkpoint. These are limited to 3.4 ounces (100 milliliters) or less per item. For some reason this is known as the 3-1-1 liquids rule and it is actually almost exactly the same as the EU rule with the same exceptions for medications, etc. I am not sure why the OP thought that the TSA rule permits two bags of liquids probably because no one ever called him or her on it and also not sure why he or she thought TSA rules would apply at LHR (even though in this case, they are the same) but no doubt lesson learned.

Thanks to the previous posters for the citations, both of which seem to indicate that medications need not be in the quart/liter bag. That is not what I was told at Heathrow, however, and the screener knew they were medications because I pointed that out. (And they looked like medications, in their original containers.)

I have not ever run into this in the US since I have always been able to fit everything into one bag (not having bought a bunch of mini-mustard jars).

So it seems like this is one of those situations in which it is best to take the conservative approach and treat all liquids/creams/gels the same way, making sure everything fits into a single bag.

I am not sure why the OP thought that the TSA rule permits two bags
of liquids

JHK, the TSA does allow you to bring liquids outside of the 3-1-1 bag if they are "medically necessary". So the OP removed their medical items and were using the extra space in the liquids bag for their mustard jars. They figured a similar exemption would be available outside the USA.

https://www.tsa.gov/travel/special-procedures

3-1-1 Liquids Rule Exemption

You may bring medically necessary liquids, medications and creams in
excess of 3.4 ounces or 100 milliliters in your carry-on bag. Remove
them from your carry-on bag to be screened separately from the rest of
your belongings. You are not required to place your liquid medication
in a plastic zip-top bag. If a liquid, gel, or aerosol declared as
medically-necessary alarms, then it may require additional screening
and may not be allowed

This could be a problem, if carrying prescription liquid meds for a long period of time is not permitted. I have flown into and out of Heathrow with liquid meds in a separate bag with no problem, but the prescriptions were in the bag as well as the medications themselves.

@ JenC, I am quite aware of the exceptions (in fact I mentioned them in my second post in this thread). I was not being clear apparently but what I was trying to get across is that in both the US and the EU there is a one-bag rule with exceptions for medication and other items. Those exempted items do not have to be in a plastic bag. So you can have one bag of liquids that are not exempt liquids -- that is the rule. You could put your exempted liquids in a bag if you are so inclined but you can't have two bags of non-exempted liquids and that is what I was trying to get at.
I note by the way that acraven says medically-necessary and not prescription liquids and creams and it could be that the guard did not recognize these items as prescription drugs that are exempt from the liquids rule.

Yes, JHK, one of my items was non-prescription (though it required a prescription 2 or 3 years ago). It may be the case that if I had buried the two items in my bag, they would not have been noticed, but I wouldn't want to do that since it would take so much time to find and remove them if they were questioned. The moral of this story is: Don't buy a bunch of mustard if you go to Dijon!

Heathrow is VERY strict. I posted a while back about having to empty my entire carry-on so that the agent could go through it because I had maybe two rogue tubes or bottles (let's say travel-size toothpaste and hotel -size shampoo bottle) that were at the bottom of my bag and not in the plastic bag. It was embarrassing, and I kept apologizing to the man behind me in line. It wasn't just, "M'am, these need to be in your plastic bag, put them in now." It was, empty the entire backpack.

I could've screamed, but you do what you have to do. Lesson learned for me for next time.

"Don't buy a bunch of mustard if you go to Dijon!" :)) Oh no, the shops in Dijon would be very sad to hear that :))

The moral of this story is: Don't buy a bunch of mustard if you go to
Dijon!

Or....check a piece of luggage so you can bring home as much mustard as you like! :-) I bought several jars of preserves when I was in London about 10 days ago. I wanted them enough to check my luggage on the way home!

To be clear, I did check a bag; my bag is never quite small enough to carry on. But I was a bit nervous about the glass jars of mustard. I figured they'd be much less likely to break in hand luggage, so I decided to take full advantage of what I thought was the rule (medications don't count). There was no reason why I couldn't have wrapped some of those jars of mustard in clothing and put them inside the checked bag.

Acraven,
I was randomly selected for an extra security check when I connected in Heathrow Airport recently, and my prescription liquid medications (eye & ear drops) were taken out of my medicine bag and placed into my 3-1-1 quart sized bag holding my toiletry liquids. The security agent at Heathrow told me that all liquids, even liquid prescription medications, must be in the same quart sized bag containing all other liquids.

It appears that this security rule is not enforced at other U.K. or European airports because this is the first time I've had to deal with this.