We will be arriving at Heathrow at 4:15 p.m. on Tuesday, June 24th via a direct flight and will be taking a connecting flight later that evening to Amsterdam. How much time should we schedule between our arrival and the connecting flight since it won't be on a partner airline so we'll need to clear customs and also check in for our connecting flight? In addition, which would be faster or easier to get to from Terminal 3 at Heathrow - Terminal 1 or Terminal 5? Thanks.
I would say two to three hours minimum. Despite the lack of a partner you might check to see if you can link your itineraries, since most major airlines will check your bags through if you ask. As of changing terminals, T3 to T1 is definitely easier than to T5. At Heathrow Terminals 1, 2 and 3 are all pretty close together (and I think are actually connected underground) with 4 and 5 farther out.
Three hours minimum.
It's not customs that is the problem it's passport control. It shouldn't be too bad on a Tuesday afternoon but T3 is the slowest of the terminals to get through. It also has the slowest baggage reclaim. The good news is that it is relatively easy to get to T1, if you're fairly fir it is often quicker to walk through the covered walkways than take the courtesy bus.