Hello! Flown into LHR many times but transferring between terminals will be a first! We come into Terminal 5 from Rome and need to get to Terminal 3 to catch the National Express Bus to Southampton after collecting our checked bags. Is this pretty straight forward and well marked? Also are there places to grab lunch after we get our bags? Thank you!
You can take Underground (the Tube) from terminal 5 to 3 for no cost. You do need a "pass" to get you through the gates, there should be directions at the station.
For food, probably more selection in the landside area of terminal 5 that you will pass through (though I have not been to that terminal) but near the bus station are a couple options. The bus area is not so much in Terminal 3, but in the area between Terminal 2 and 3.
The way, as I recall is well marked, or at least I found it without any problem from Terminal 3.
In addition to the Underground, you can also take the Elizabeth Line or Heathrow Express for free to transfer between terminals (the carriages are bigger than the Underground and a bit easier to manage with luggage). There are a few options to eat a both terminals that don't involve going through security- at T5 they are alongside the check-in desks and at T3 you need to go upstairs. If you can find where the Virgin check-in desks at T3 are and take the escalator up from there, then turn left at the top of the escalator, there is a pub that serves food.
Have you checked to see if your bus stops at T5?
All Heathrow to Southampton buses call at T4 and T5 as well as the Central Bus Station for T2/3. So no need to switch terminals.
https://timetables.nationalexpress.com/routes/NX/205/Heathrow_Airport-Poole/O