Just wondering how difficult it is to transition from t3 - t5 - Do you need to clear customs and immigration check between terminals? Is there transportation within the airport internal facilities ?
It depends on your ticket and luggage. If you have bags checked through, or have carry-on only, you can use the "Flight Connections" path to transfer Irside between T3 and T5, without going through passport control. You follow the purple Flight Connections signs and there will be a dedicated bus (free) that takes you between the terminals. It runs about every 10 minutes. You will likely go through security again.
But if you have to claim and re-check bags for your onward flight, you will have to queue for Immigrarion-Passport Control to "enter" the UK, and go to baggage claim to pick up your bag. You can use the free Heathrow Express to transfer between terminals, re-check your bag, and then gomthroughnsecurity and on to your gate. This will take a lot more time than the airside transfer.
Heathrow's website has a wonderful connection tool. Put in all your details, and it will tell you all the steps and estimated time needed: http://www.heathrow.com/flight-connections
Yes, it is a good tool, but pay attention to the caveat that the minimum connect times only apply if you are traveling on one "through" ticket, and baggage is checked through.
If your connecting flight is to somewhere else in the Common Travel Area (most likely elsewhere in the UK or Ireland) you will need to go through immigration although you do not need to collect any through-checked bags.
Even on an airside transfer you will go through security as the UK doesn't take on trust anyone else's security including the USA.
From personal experience (several times), it is not difficult but it can be a little intimidating and fatiguing, especially if you have mobility issues or have a tight-ish connection time. You go through an abbreviated passport type security line where they make sure you have the proper documents, ride a bus to terminal 5, and then you go through UK airport security (like our TSA). Same security rules apply PLUS all electronics must be run through separately and each must have enough battery power to boot up (although I've never witnessed them ask to turn one on). Also, the UK has similar limits on liquids, pastes and jells so have them in a quart bag.
Bottlenecks in this process are at the entry control queues, bus loading stations and security (of course).
If you, or someone you are traveling with, have mobility issues, ensure the airline meets you ON THE PLANE with a chair. It is my opinion that Heathrow is the very worst airport when it comes to assisting disabled passengers. On three visits traveling with a disabled passenger, they failed the minimum standard...
The post by "worktotravel" about needing assistance ar Heathrow made me remember the time we arrived from Rome at Heathrow and I was still suffering the aftermath of serious sea sickness. A wonderful turbaned man met me at the plane, took me through security, etc. and insisted on taking me right to the car of the friend who was meeting us. He was so attentive and kind.