Transfer at Heathrow

If you are doing this on one ticket, your luggage will be checked through to your final destintion.
If you have one ticket (do not need to check in again) you will stay "airside" at Heathrow and not need to go through British Immigration.
You need to check if you are arriving and departing from the same terminal. Thta will make a big difference to the amount of time needed. Heathrow is effectively 3 airports:

• Terminals 1, 2 and 3 are together in the centre of the airport.
• Terminal 4 is at the south side.
• Terminal 5 is at the west side.

For more info see here: http://www.heathrow.com/flight-connections

BA flights from Barcelona presently arrive in Terminal 3 and flights to Toronto depart from Terminal 5.

On arrival at Terminal 3 follow the signs to flight connections - do not go through immigration. You will take the airside bus to Terminal 5 where you will go through security and enter the departure area.

Four hours is ample time for such a transfer. The minimum connection time is 90 minutes.

Check your bags through to your final destination.

Great, thanks! I thought for sure I'd have to go through immigration bc of the terminal change. This is much easier.

Another thing to remember, their security rules are a little different than ours. They have the same 3-1-1 rule, but ALL electronics must come out and be capable of being powered on on-demand (so, make sure all are charged).

Look on Heathrow's web site for security rules and take a close look at the posted signs while in the security screening area. While I have found Heathrow security personnel to be more personable than our average TSA personnel, I also have seen them a lot less tolerant of misbehavior (which isn't necessarily a bad thing).

And, remember, TSA Pre-Check doesn't do much for you overseas.

We were flying from LAX to Zurich, starting on United to Heathrow and then Swiss to ZRH. Even though we never left Terminal 4 and had no access to "unsecure" areas, we went through a British security screening. As noted, they were very efficient, very friendly, lots of ziploc bags available. My rule is to overdisclose - if they want liquids over a certain size, I'm just going to show you ALL my liquids so you decide.

Also, Terminal 4 is HUUUUUGE as a certain candidate would say. It has little direction signposts like you'd see in a Swiss village with walking times to your destination!

Last year I connected thru Heathrow on way back to U.S. Landed at Terminal 5 and departed from 3 -- I was surprised at how long it took ... And everything went smoothly - it just took a great deal of time to walk (and I'm a fast walker!) to where buses were, then 10 minute + wait for bus, 5-10 minute bus ride, 2 different checks of carry-on luggage, then honestly don't recall if it was immigration or not, but another check for passports, then a super long line to "check in" for flight (FYI: I'm a U.S. Citizen). It took just over 2 1/2 hours -- thank god I had 3 for my connection.
I have to connect thru Heathrow this fall coming back from Munich, and months after booking flight, the BA flight changed so we'd only have 1:50 to make connection -- while Heathrow site says 90 minutes are enuf, BA site says at least 2 hrs. Needed, so I changed to a later flight out of Heathrow.
I think you'll need a good chunk of your 4 hours.

Within Heathrow, if the airline will sell you a connecting journey then it should be long enough - if you mis-connect it will be their fault not yours providing you haven't spent a long time in the bar in between I suppose.

The CAA doesn't take on trust any other country's security so you will always need to go through that when connecting.

Always try to ensure that you are already checked in to your connecting flight as far as possible.