LHR airport and connecting flight to Madrid

The general rule of thumb is that for two separate flights you should leave at least 4 hours, so neither flight could be recommended. However, if you can check in to the second flight and have your details with you when you land and you only have carry on luggage, you should be able to use Flight Connections and travel between terminals airside. This should mean you will be OK but I would still book the later flight.

Using Flight Connections means that you will not enter the UK and will therefore not need to get yourself an ETA. But, given the circumstances and the fact that an ETA only costs £20 even with the price rise I would probably recommend that you get you one anyway in case the first flight is delayed. A delay that means you miss your second flight will of course mean that you lose it and will need to buy a new one at the walk up price and that any return ticket will also be lost, but I assume that you understand that.

Check which terminals you will be arriving to and leaving from. Virgin Atlantic arrives into Terminal 3; BA & Iberia both leave out of Terminal 5. The bus between those 2 terminals (which I took 3 weeks ago) takes roughly 30 minutes. Since we switched terminals, we had to go through Security. Furthermore, it took us 20 minutes to get from the Flagship Lounge to our Gate - the terminals are huge. Even assuming that the VA flight arrives on time, I would not feel comfortable with a 2 hour window. The 6:15 flight would give you more of a cushion, and it is just 1 hour later than the 5:00, so to me, it's worth taking the later flight and not having to worry about connecting.

I would definitely leave more than two hours- mostly because there is no guarantee your first flight will arrive on time, and since you're booking them separately you won't be automatically rebooked if that causes you to miss your second flight. Heathrow is huge and can take a very long time to get from one gate to another.

I certainly wouldn't risk two hours, I might take a change on 3 1/4. Others have pointed out you need to get through immigration in Terminal 3, then out to terminal 5 - even if the plane is on time, and you get lucky with timings, it still is unlikely you will have enough time to get back through security and to your gate in time (which can be twenty minutes from security).

I'll add a further complication, the last two trips I've made on Iberia, they have sent me an Email requesting that anything bigger than a smallish backpack that fits under the seats in front (16x12x6 inches) be checked in. Flights on bigger aircraft (e.g. the Airbus A-330) also start boarding 40 minutes before departure.

"Others have pointed out you need to get through immigration in Terminal 3, then out to terminal 5"

No you don't. As long as you only have carry-on luggage and your onward boarding pass you can use Flight Connections and avoid immigration. You will still need to do security in T5 though.

While technically you do not need an ETA for a transit, the problem you will likely run into is that as far as Virgin Air knows, you are only going to London, so they may demand an ETA and check for it. Possibly, if you show them the same-day onward ticket, they may relent, but they may err on the side of caution and deny you boarding without an ETA.

Be aware, the use Flight Connections at Heathrow, you will need the boarding pass for your second flight. Downloading it onto your phone is fine. Flight connections is easy. Follow the Flight Connections signs at T3 for the shuttle buses. Get in line for T5. You will be shuttled there. Upon arriving at T5, you will go through security and then are free to go to your gate when it is announced.

So many people don't really understand what an ETA is. It's an Electronic Travel Authorization. It is authorizing you to travel. To get on the airplane. It is basically the first level on checking to see if you qualify to enter the UK. It doesn't guarantee you admission, it just allows you to get on the airplane. It is not a visa.

If you don't have plans to enter the UK, then you don't need one. Technically you are in "no mans land" at Heathrow and the UK is not interested if you are able to enter the country.

Now, should something happen and you have to stay, they will process you just a little differently. They will not deny you admission because you don't have an ETA, they will not throw you in prison because you don't have an ETA, they will not make you sleep on the tarmac because you don't have an ETA.

If Virgin Atlantic asks about it, and I doubt they will, just show them the ongoing ticket or boarding pass proving you don't plan to stay in the UK and are just passing through.

"As long as you only have carry-on luggage and your onward boarding pass you can use Flight Connections"

I stand corrected.

To jump on the ETA scenario, I think it would be wise to get an ETA.

From Virgin's point of view, you're travelling to the UK. What you're doing after that doesn't really interest them. Virgin might deny you boarding if you don't have the ETA. To be told that at the gate on departure might be a tad upsetting, and by then it's too late to apply.

Thank you, everyone for all of the advice. Especially about the ETA. I didn't think about that, so we will make sure we get that well in advance of our trip. Just going to take the later flight and not have the stress of the tight connection.
Thanks again!

I just started looking into this and there is the ETA and then there is the ETIAS that starts the last quarter of 2026. We will be traveling September 20-October 6th and plan on spending the night in London on Oct 5th and heading back to US on the 6th.
So, does anyone have any info on ETIASS an if I should wait?

Never mind. I just looked it all up. I may end up needing both. Won't know until ETAIS is officially up and running.

ETIAS will be the EU equivalent of the UK ETA. It is scheduled to be introduced later this year but will very likely be delayed. The first stage, EES, should have been fully in operation by last week but has been further delayed by 6 months as some countries are not ready. There is nothing for you to do re EES - it is the collection of facial and fingerprint information which you should do on arrival in Madrid in your case unless you have already done it. Just keep your eye on the news re ETIAS but I would be very surprised if it’s in place by the time you travel.