This is probably why I'm seeing travelers fret about having to switch their items from their own clear travel bag to LHR's--the zipper/ziploc part. Thanks!

Thanks for the responses! I'll probably just use the liter ones I got since they have a teeny bit more space than US quart bags but nice to know it's not a must. I did notice that most of the complaints about LHR security being difficult (mostly on other travel boards) were from people who had forgotten a loose bottle or tube of something in their carry-on and then security got tough. Of course people usually don't post when everything goes smoothly...

Thanks again!