Hello All,
I'm embarking on my first trip to London in a few weeks. My flight is going to arrive early in the morning but I can't check-in to the room I'm renting (via Air BNB) until late afternoon. Are there train stations or popular areas that have luggage lockers or luggage leave services? I'm having quite a bit of difficulty locating this sort of information. I'd hate to waste a day just because I'm saddled with a suitcase.

There are luggage consignment places at all central London train stations and lockers at the London Coach Station in Victoria.
If you tell us which airport you're flying into and which area you'll be staying in we can be more precise.

There are luggage consignment places at all central London train stations
Not all, just the biggest ones. Just Charing Cross, Euston, Kings Cross, Liverpool St, Paddington, St Pancras, Victoria & Waterloo.
They are very expensive and charge per bag.
http://www.left-baggage.co.uk/locations.php
An alternative is to leave your bag for free in one of the free museums in the city. But they won't appreciate holding a huge amount, and you might want to consider making a large donation to the museum.