Hello Jensy, when you arrive at Heathrow after your connecting flight, all you should need to do is to go from the arrivals area to the departure area and find your gate. You will receive boarding passes for both your flights when you check in at Newcastle, and your luggage will be transferred.
If you still do not feel confident, remember that you will be able to meet up with the rest of your holiday makers. You will all be together and you will all manage to make it onto your departing flight.
http://www.heathrow-airport-guide.co.uk/
for assistance, or ring the Heathrow Airport Customer Service line on 0904 2562224.
I hope this information helps, and if you have any further queries then feel free to email me.
kind regards
James, your editor