As Canadian citizens you can enter the UK without visa, unless there are special reasons why you in particular cannot do so.
If you have been in the UK recently for a long time or have been traveling in and out of the UK several times in the last few months you may be stopped, although this is not always the case.
And if you have reasons to apply for a UK visa (like having been denied an entry into the UK in the past) you better plan on not doing it.
The thing that would stop most people from just popping out of the secure area is time. It takes time to pass through immigration, how long depends on many things like how many planes have landed just before yours. And on the way back in you will need to pass through security, which can take time again.
5 hours should be plenty, but why risk it? And why queue for a long time, it can be a few hours in the end, just for passing time?
Side note, you may have to pass through security in any case, depending on where you come from. I am not sure, but I think that coming in from selected European airports you may not need to.
And you might find the thing you want to eat in the secure area of the airport. I have not been in Heathrow recently but in the UK airports I use there are several food outlets of the same kind, not sure about the brands.