Even if Canada is not as strict as the US, the rules are pretty much the same. So you need to be well prepared and make sure all your documents are in order.
British citizens do not need a visa to visit or transit in Canada, if you hold a British passport issued by the United Kingdom, which contains the observation that the holder has the right of abode in the United Kingdom. Your passport needs to be valid for the entirety of your stay. As from November 2015 all visitors need an Electronic Travel Authorisation (eTA). You can apply online for a 7$ (Canadian) fee and it's valid for 5 years.
Take out comprehensive travel insurance - medical fees in Canada are expensive. Note down where you intend to stay (hotel, youth hostel etc.) as you may be asked for it on arrival. To make life easier, it's best to have a return ticket that you can produce. If not, you may be refused entry.
Before you pack, check what food or items are permitted to be brought into Canada. If you're not sure, contact the airline, travel agency you're using or go to the Canadian government's website.
You can't, for example bring in fresh cheese or conserved meat (sorry, Cheddar's out) … However, rules can change so check out the Canadian Food Inspection Agency's website. You can go Duty Free shopping in the departure lounge; limits are one bottle of strong alcohol, two bottles of wine and 200 cigarettes. Any food or vegetables not on the government list must be declared at customs. The website will also tell you how to get relevant permits. They'll probably say nothing if you've got a packet of Digestives but your 'can't live without streaky bacon' may well get confiscated!
Before you land you'll be given a form to fill out, detailing the reason for your trip, hotel address, your travel itnerary for Canada and what you have in your case.
At customs, you'll be asked some routine questions to make sure you're in line with their rules. Be warned, if you land in Montreal they'll ask you in French. But you'll be fine in Vancouver - questions will be in English!
If you have declared food or plants then your bags will probably be searched. But that's normal practice - they're just doing their job. Whatever you do, don't tell fibs! Customs officials don't mess around and you'll be slapped with a fine! If you're taking in medicines, make a copy of your prescription and get it signed by your dispensing chemist.