Jfk-lhr-jnb
 

We'll be flying JFK to JNB with a change of planes and 8 hour layover in LHR. Will we have to clear UK customs or is there a transit area we can wait in?

You can stay airside by following "flight connections", avoiding the need to clear customs and enter the UK. Airside includes quite a few shops at LHR, particularly in T4, so maybe it is the most pleasant airport to do this at (however you can only be airside at the terminal from which your flight to JNB departs, which is T1 if flying BA or T3 if flying most others). But 8 hours is a lot to stay at an airport, particularly with 8+ hour flights before/after. If you have no airline lounge access, there are several "paid-for" lounges available at LHR for a fee.

You can cool your heels for 8 hours in T1 (as number_6 notes, it's good but not as good as T4) or, my choice: go through immigration, park your bags at Left Luggage (£5 per bag, in the arrivals hall) and have a day in London via Heathrow Express. It's not terribly cheap--it'll cost you about $50 per person for the left luggage and train fare alone--but at least you'll have some fun and a nice lunch.

Take the Tube (subway) to Central London. It's much cheaper than the express--you have 8 hours to kill anyway. Also, if the OP is connecting at LHR, wouldn't the luggage be already checked and in possession of the airline? Would there still be the need for luggage storage? Just wondering.

LAX

or, you could do what i did in january. i booked a day use room at the london heathrow sheraton. i had a shower and slept for a few hours. it was great. and i was refreshed for the long journey to jnb.

I am not a fan of T1 and if taht's where you're headed I strongly recommend either going into London (I'd take the underground--you won't save THAT much time by taking HEX unless you're planning activities in the Paddington area) or the airport hotel option. If you are unable or unwilling to check your bags through, the left luggage charge is now 5.50 GBP per bag.

If you choose to do this, there's no necessity to collect your bags. Just make sure they're through-tagged to JNB and they should be transferred automatically. Then you can go through immigration and waltz straight out through Customs and get going into London. (And you don't have to pay the Excess Baggage Company.)