I am arriving at Heathrow Airport on Terminal 5. I intend to get to Cambridge. Questions a) I have a Britrail point to point ticket. Is it valid on the tube as I gather the best and fastest way to get to King's Cross is by tube? b) Or is it better to take the Piccadilly line from Terminal 5 to King's Cross? c) I believe to get to Cambridge I will need to take a train from King's Cross, Is that right?
Firstly, your Brit Rail point to point ticket is NOT valid on the London Underground. From BritRail.net:
The BritRail ticket or pass is not valid on Eurostar services, special excursions, Tyne and Wear Metro Trains (Newcastle-upon-Tyne area), Glasgow Underground trains, Manchester Metro link, road links between airports and railway stations, Docklands Light Railway, privately owned railways, steam services, London Underground trains and buses, ships, hovercraft, nor any other services not mentioned.
So instead, you have a few options. Firstly, it's worth noting there are actually direct buses from Heathrow to Cambridge:
Coaches leave hourly. Allow up to three hours for the journey. One-way ticket £28.20 or £29.20, return ticket £36.50.
However, if you want to take the train and tube combo, you have two options. You can take the Heathrow Express to Paddington (valid with your BritRail ticket), or the Picadilly line to Kings Cross (25 stops, from memory, it's a long way).
There are also regular trains from Heathrow to Paddington.
If you take one of the Paddington options, you can then get the Circle or Hammersmith and City line around to Kings Cross.
From Kings Cross, you'll need to head to the overground station to get trains out to Cambridge.
If you want to plan the journey and see how long this might take you, have a look at the National Rail website where you can plan your journey and see the durations and costs of each leg.