At what point do I go through customs?

Since England isn’t in the Schengen Zone, you’ll go through immigration in Spain (your first stop in the zone).

You will definitely need to go through security again in London,

Thank you! Since it is two separate tickets I will plan on customs for London. Confirming I would get our luggage off the baggage terminal (Terminal 5), go through customs, then rebook the bag for the flight to Barcelona. Thank you!

Since your flights are on separate tickets, your checked bag may not be checked through, but it may be possible since both flights are with the same airline. If not, you will first need to pass through UK Border Control. And you will need to have an ETA. Then you can pick up your bag, go through Customs, check your bag and get your boarding pass at the BA counter, go through security, and proceed to your gate. I hope you have left sufficient time to do all this.

I think the confusion may be on the term “separate flights.” Do both flights have the same “confirmation code” or were they booked at different times and have different codes? If so, you need to check with BA about whether your flights can be linked and checked bags forwarded. Otherwise I hope you have a large window between flights as the process to go back thru, as described above, will be quite lengthy.

CJean made an important point, which you definitely need to plan ahead for.

Since you are on two separate tickets and will have a checked bag, you will have to exit the "airside" at Heathrow and go through border control. That will require you to get an ETA (electronic travel authorization) for the UK, which you need to do in advance. Then you will have to go to the BA counter to check your bag before you can go through security and reach the gate for your second flight.

If you can find a way to travel with carry-on only, you will be able to transfer and remain airside as long as you already have your boarding pass for the second flight.

I suppose it might be possible for them to check your bag through, but normally that only works if you're on a single ticket. I'd call BA and ask beforehand. I tend to doubt they'll do it.

Janey, it might help if you understand the correct terminology.

Immigration/border control is where you are admitted to the country and passports are checked. Customs is strictly about the goods (your baggage) you are bringing in.

Hopefully, you've allowed plenty of time. Have a great trip!

Unfortunately, for Janey British Airways is not one of the carriers that will through check luggage on separate tickets unless you get incredibly lucky. Delta does it routinely. British airlines policy is it is not doable. They want to discourage this.

Hopefully, you have allowed a lot of time to do all this

First thing....call BA and see if they will combine your two flights into one single reservation. (One single reservation can more than one flight.)

If they do that, you will be able to check your bag through to Spain.

If they won't, when you arrive in London, you will have to go through immigration--make sure you have an ETA--then pick up your bag and exit through customs. Once through customs you will recheck your bag, go through security and go to your gate.

Curious why the OP did not just buy a ticket from home airport to Spain destination through BA. That would have made life so much simpler.

Thank you all for your assistance.
Spain was added on to our itinerary after we purchased the London round trip ticket. The flights coincidently ended up being British air, Terminal 5.
If it was me alone, I could definitely do just a carry on, but my husband does his photography equipment as a carry on and will need to check a bag. The trip is 3 1/2 weeks altogether.
We have 4 1/2 hours between arrival to London and departure to Spain. Hopefully we will be ok!

4.5 hours in London should be plenty of time as long as you don't have a lengthy delayed arrival.
Border Control is fast. As long you have your ETA you simply use the e-gates (scan passport, have your picture taken) and you are through. Claiming luggage can take the time, but I've never had it take more than an hour.

When you exit arrivals at terminal 5, just take the elevators up to departures. They are automatic and are much quicker than the escalators. You will go through an automated bag drop, have your passport checked and then head through security. Security at Heathrow terminal 5 no longer requires you to take anything our of your bag. You will need to empty your pockets completely! Once through, check the departures board. Don't be surprised if your gate isn't shown yet.

Worst Cast scenarios once the plane has arrived at gate:
Walking to Passport Control, 15 minutes
Passport Control, 15 minutes
Luggage collection and additional 45 minutes
Bag Drop 15 minutes
Security 15 minutes.

I would expect that from the time you leave the plan to making it through security will probably be no more than 2 hours at most.

That is so helpful Carol. Thank you ever so much! J. I’m making notes for everything you shared.

Departure gates at Heathrow are posted about an hour before departure. The departure board may even have a note stating the time the gate will be posted.

Depending what time of year this trip is, weather could be a factor at your origin or London. And the farther in the future the trip is the more time there is for the airline to tinker with the schedule, which may not work in your favor

Barring weird or ugly happening, as already noted, the 4.5 hrs should be fine

Comment about airlines “tweaking” the flight details is completely valid. I can’t think of any flights I’ve taken within the last 3 years, USA or abroad, that have not made at least some kind of change . The trip is 6 1/2 months away (mid April). Hopefully, nothing too terrible will happen! We did purchase travel insurance.

If you download the Heathrow app and set an alert for your flight to Spain, you might even get the gate info before it's announced in the airport; I really found the app very helpful.