You have a visa for the UK, but want to arrive before its start date.
The controlling technical reference for your question is found in Paragraph 30C of the Immigration Rules, and it says...
30C. An Immigration Officer may cancel an entry clearance which is
capable of having effect as leave to enter if the holder arrives in
the United Kingdom before the day on which the entry clearance becomes
effective or if the holder seeks to enter the United Kingdom for a
purpose other than the purpose specified in the entry clearance.
There's some key words here...
"may cancel", as opposed to "will cancel", the option to land you is entirely within the Immigration Officer's discretion. If he wants to, he can annotate your entry clearance and land you under a Paragraph 31, that's what it's there for. However, they tend to see this situation as an individual attempting to get more time than he was granted (which it is by the way) and that's abuse. So the odds are that the IO will not exercise discretion in your favour. That comes from about 18 years of watching them where I have never seen an IO use Paragraph 31 or 31A where the person was making it up as they went along. But there can be happy endings, more about that below.
Another key phrase is "for a purpose other than the purpose specified in the entry clearance". This is dangerous ground and if the Immigration Officer suspects that you are up to something that you did not disclose in your application, he can use Paragraph 320 to have you removed. This is a very bad outcome and should be avoided. After that happens you can forget the UK unless you instruct a solicitor. You can read where another unlucky fellow got slapped with Paragraph 320 here: Refused Entry and Removed from the UK; What are the Consequences?
Having said all of that, these situations are wholly governed by personal impact and articulation skills. That means if you are able to give a coherent and rational explanation in a way that convinces the IO that your early arrival makes sense, he will use the rules that allow him to land you, no problem. The caveat is that you've got to be really good at it.
For your other questions...
Can I stay at the airport for more than 10 hours and enter the UK
after my visa begin to be valid?
This is an emphatic no. There are no facilities between the gate and the primary control point for an individual to 'hang out'. In fact the furniture is sparse to none. If you try to hang out in one of the vacant gates, you'll be spotted by a camera and matters will be worse.
The Primary Control Point (PCP) refers to the immigration control area
(arrivals hall) where passengers present their travel documentation
and may be questioned by Immigration Officers about their reasons for
entering the UK
(The Chief Inspector's Audit of Terminal 3 Heathrow)
The whole place between stepping off the plane is a controlled area focused on people reaching the primary control point. You'll find toilets and little else.
If that won't work, can I buy a ticket leaving on June 24th and arrive
on June 25th in UK?
Of course, as long as your arrival in the UK is after midnight on the 24th, it's fine.
Some respected regulars have already pointed out in comments that you may even face difficulty boarding the plane in Canada.