Planning a trip to Canada/US. Can I get US Pre-Clearance in Toronto for Land Crossing?

To clarify the situation: the Toronto airport is nowhere near the border. Although there are pre-clearance facilities for flying into the US (with US agents stationed in Canadian territory), that's not what you intend to do here.

When you fly from the UK to Toronto, you will go through Canadian immigration and customs. You will not be interacting with American agents. To speak to an American agent, you would have to be transferring to a US-bound flight.

Even if you somehow weasel your way into the US pre-screening area without showing a valid boarding pass, what good would it do you to enter the US side of the airport? That's not where you want to be. Getting screened to enter the US at that point would just waste time and raise suspicion.

So, just go through Canadian formalities at Toronto to enter Canada, and go through US formalities when driving to Buffalo, like a normal traveller.

I think I would add that a Global Entry membership is about as close as you can get to US "immigration pre-clearance" in the sense you seem to be using the term (which isn't how the CBP uses the term). Unfortunately this only helps non-US people when travelling by air; having a Global Entry card allows one to use the NEXUS lanes at Canada land border crossings, but I understand they only issue the cards to US citizens and permanent residents and to Mexicans.

Also, unless things have changed very recently, the land border still operates with paper I-94's so your visit to the office will likely include buying one of these. Take some US cash (are they still $6?). Once you have the I-94 I think you can use it for its period of validity if you want to cross the border again, and that might keep you out of the office on subsequent crossings.

Be sure, however, to surrender the form before its expiry once you are done with the trip. I think you can ask the CBSA officer you see at your last land crossing into Canada to take it or, failing that, turn it over at one of the CBSA offices at YYZ. If you leave with the I-94 or you didn't have one when you left the US via the land border (which might happen if you fly into the US but fly back from Canada or Mexico) the departure likely won't be recorded in your I-94 history and you'll need to fix that by mail. If your timely departure isn't recorded they'll give you a hard time over the apparent overstay on future visits to the US.