Hi Ross,
I can share my experiences, although there is no guarantee.
I have and use the clear, plastic, cosmetic cases you refer to. They are a wee bit bigger than a regular zip lock bag. They have the zipper and are more durable.
Many stores sell those now --- TJMaxx, Marshall's, Bed, Bath & Beyond and even Walmart and Target (even including the various sized jars and bottles.)
They do state "TSA approved." In the last couple of years, I have seen those bags from passengers being put in bins.
I have flown through Heathrow a number of times and was never questioned. But, if you are a bit nervous, keep a couple of zip lock bags in the front pocket of your luggage. The bags weigh nothing and you may need them on your trip elsewhere. I was never questioned at Logan either. (I also traveled Domestically on a couple of trips last year and was never questioned.)
I carry on and check. So, I split my toiletries accordingly. All my containers are not the full 3-oz size. I can fit a lot in the bags. If you are creative and good at decanting and organizing, you can take your favorites for your trip.