I am traveling to the US today, but my Canadian passport expired last month. Can I use my British passport to travel ?

You can use the British passport to travel as long as it has, I think 6 months left on it, but don't you now need to do a new ESTA for the different passport?

Someone more savvy than I am will likely be along to confirm, but I'm 99% sure you'll need to redo ESTA.

My British passport expires next year, so that should be fine? And yes, I'll have to renew my Canadian one asap.
Thanks!

The passport itself is OK - if you're British and entitled to have it you can travel on it.

If you are entering on the VWP, you will have had to complete ESTA - but did you have to enter your passport info when you did that? If so, then you'll need to do a new one for the Brit PP. You'll have to excuse me, I've never had to use ESTA as it's relatively new.

ESTA only required if travelling by air. OP does not state method of travel.

I'm British, but I have never used it before so I don't really know anything about it.
I've never had to use ESTA before either.. Would I have to do it/have time if I am traveling later today?

Oh! I'm driving to Buffalo..

You don't need ESTA, then. Just a few bucks and a UK passport should work.

True. I thought it prudent to mention but in this case it was irrelevant.

There isn't a 6 month requirement, would only get stamped for the length the passport is valid though if it were less.

Not much help, but i renewed in St. Catherines in one day.

My condolences.

for what...?

Buffalo

Not so for UK passports.