My husband and I are considering a travel itinerary from NYC to Athens with a connection at Heathrow. We are booking the flights ourselves on two different airlines and would like to know (a) if we've allowed enough time to make the connection and (b) if we will have to go through customs/immigration at Heathrow. We are inexperienced travelers and really don't know what to expect!

We are scheduled to arrive arrive at Heathrow at 8:50 a.m. on a Saturday on Virgin Atlantic (Terminal 3) and depart at 12:20 p.m. the same day on Olympic Air (Terminal 2). That's 3.5 hours between flights.

Any advice would be greatly appreciated!

Went thru Heathrow last June enroute to Cork. Did not have to claim baggage but did have to go thru customs and show passport and other documents. It was not a big delay. Getting around Heathrow can be a bit dodgy but t is well signed

You should be fine - just be sure when you check in that Virgin is able to tag your bags through to Athens - they ought to be able to do that. You will clear customs at Athens, but you may need to go through a security check at London (metal detector, etc.)

Transfer from T3 to T2 is no big deal - you have plenty of time. If for some reason Virgin can't through-check your bags, then you will still have adequate time.

Some airlines are getting increasingly unhelpful about through-checking on two tickets.

All departing passengers from LHR, wherever they've come from, have to go through security again.

If Virgin will through-check your bags, you simply follow the signs for international connections at T3, go through security and get the bus across to to T2. 30 mins is more than ample. You go through neither immigration nor customs.

If they won't, you have to clear immigration at T3, collect your bags, put them on a trolley, take them through Customs (never an issue in Britain), push it the 400 yards or so along the underpass to T2, check in with Olympic then go through security. Last time I had to do this - admittedly with a UK passport - it took 45 mins from getting off the transatlantic flight to sitting with a drink in a T2 airline lounge, having cleared security. Even under the worst circumstances, I can't see it taking more than 90 minutes from arrival at the gate to getting through T2 security.

You are traveling on 2 non-partner airlines. Virgin will NOT issue the boarding pass for Olympic when you check in for your initial flight. So you will have to get your boarding passes for Olympic once you arrive in LHR.

Unless there is a way for you to get your boarding passes without going to the Olympic Air departures desk, you will have to go through immigration to get to the departures desk.
If Virgin will through-check your luggage, you shouldn't have to collect your luggage. However, be careful with this part. About 3 years ago, I switched airlines with my luggage supposedly through-checked. However, when I was getting my boarding pass for the second airline, I asked about luggage to check in. At that time, I was told that I would have to collect my luggage and recheck it! And sure enough, my luggage was on the luggage carousel! (This did not happen at LHR and did not involve wither of your airlines.)