Thanks for the replies all. I was aware US didn’t really recognise the concept of transit, but assumed this was just for the purpose of immigration; didn’t realise this extended to suitcases as well! I have a valid Esta from a previous visit last October so I’m all good on that front.

Both flights are booked with a single PNR; so I’m assuming that in London I’ll get issued two boarding passes like for a normal connection. When arriving at LAX I clear immigration, collect my bag like a normal arriving customer, pass through customs then immediately after there’s a transit counter at which I drop off my (already tagged) bags. Upon my return to the airport in the evening I can just go through with my second boarding pass issued in London?