U.K. and Ireland - Will I freeze at Heathrow?

Hi all, I have a connection (and long layover) at Heathrow in a few days. I am starting in a hot climate and my final destination is a hot climate. I arrive in Terminal 3 and my next flight is from Terminal 4 (staying at the Yotel there for a few hours).

My question is - do I have to go outside to transfer between terminals? Is it a bus, or an indoor train? If a bus, can you wait inside and just run out to the bus, or is it out in the open far from shelter? And what is the general climate within the terminal? I'd rather not have to bundle up in winter clothes if I can help it, but I don't want to catch pneumonia either.

Thanks!

Unless your flight is not using an air bridge or you decide to take the bus between terminals rather than train then you shouldn't even see daylight, let alone go outdoors.

May I suggest you read the "Airport Information" section on the Heathrow Airport website

To get from Terminal 3 to Terminal 4 landside, the Heathrow Express train runs a free shuttle service every 15 minutes. You needn't go outside for this - there's an underground walkway from T3 to the (underground) station, and at T4, the station is inside the terminal. In T4, you just go up to the mezzanine level on the escalators and you are there.

While you're not outside at all, there will be open doors and breezy corridors. The weather isn't terrible at the moment - typical January: dull, grey, and between 5-10C most of the time - but some snow is forecast for the coming days.

I'd bring something you can put on in layers. Personally, I'm going outside with a simple fleece or woollen top over my shirt around this time of year, but then I'm Scottish and usually get around by cycling, so that's possibly not typical.

I'd add one more layer than that!

Unless your flight is not using an air bridge or you decide to take the bus between terminals rather than train then you shouldn't even see daylight, let alone go outdoors.

May I suggest you read the "Airport Information" section on the Heathrow Airport website
May I suggest you wait on the "Heathrow T123" platforms for a while? The draft can be unpleasant, and the station does not appear to be heated.

A light pashmina should do the trick though.

make sure its a removable layer though, as the terminal buildings can be very warm if you have a lot of layers on.

Thanks everyone, extremely helpful!