Ticket from Attleborough to Heathrow T3

Yes, it does, but you will not be able to make any extra stops on the tube. You will be able to use it to go on the tube from Kings Cross to Paddington.

I have just obtained two ticket prices from the Trainline website.

The first was from Attleborough to Heathrow Airport, the second from Attleborough only to Kings Cross station. It is obvious from the pricing and the routing that the first covers the journey all the way to Heathrow.

For Heathrow, there are two options: 2 changes of train, or 3 changes of train.

• 2 changes: the route map shows changes at Cambridge and Kings Cross, then the underground to Heathrow T2 or T3.

• 3 changes: the route map shows changes at Cambridge, Kings Cross and Paddington, then the Heathrow express to Heathrow T2 or T3.

The prices vary accordingly. My favourite route is the underground Piccadilly Line direct to Heathrow, a one hour ride from Kings Cross. The Heathrow Express is arguably quicker, but is also more expensive and you first have to reach Paddington Station. If you have a lot of luggage the Express might be easier, taking a taxi to Paddington. You can also get to Paddington by tube (which is the route given by Trainline), but then you might as well use the other route which is less hassle.

The short answer is yes. To be more specific, there are a few possibilities here depending on exactly what you bought. In short, there are three different ways to get to Heathrow by rail for your purposes — there's the main-line Heathrow Express train which runs from London Paddington station and is very fast (15 minutes from Paddington), the slower TfL Rail train which also runs from London Paddington but stops at intermediate stations and takes about half an hour from Paddington, and finally the London Underground's Piccadilly Line which is also pretty slow (it takes around an hour from King's Cross), but has the advantage of you having to catch only one train from King's Cross/St. Pancras, since the Piccadilly Line stops there; rather than having to change at Paddington. (The journey from King's Cross to Paddington itself would take only around 10-15 minutes by tube, but when you factor in the time to find the tube station at King's Cross and find the right platform at Paddington this is likely to be considerably more, making Heathrow Express still faster but TfL Rail probably not faster than the Piccadilly line overall).

So with that in mind, here are the possibilities for ticketing:

• You have bought a ticket with the destination "HEATHROW RAIL". This will have a little dagger icon (for historical reasons known to the railway as a Maltese cross) printed next to the route restriction. This symbol means is valid for one journey on the tube via any reasonable route from your arrival London Terminal to your departure one, in this case from King's Cross/St. Pancras to Paddington most likely. (If you decide to exit the tube before Paddington station you may do so but you must then make your own way the rest of the distance; your ticket will not be valid to enter the tube a second time). The same of course applies in reverse on the return leg if you have a return ticket. Once you reach Paddington you would then use the Heathrow Express to get to the airport itself; this of course is also included in your ticket price. This ticket is also valid on TfL Rail but is NOT valid on the Piccadilly Line all the way to Heathrow.
• You have bought a Travelcard. This will say "Travelcard" on it and is valid for unlimited journeys in Zones 1-6 of London on the day of its validity. This includes the Piccadilly line to Heathrow, the tube to Paddington, and TfL Rail's slower mainline trains to Heathrow (but NOT Heathrow Express)
• You have bought a combined rail/London Underground ticket. From your comments this seems to be what you are looking at. This will have a destination of something like "Zone U1256 Londn". This is valid for a single tube journey to somewhere in Zones 1-6. This ticket will be valid on the Piccadilly line to Heathrow only (not TfL Rail or Heathrow Express). The Piccadilly Line is the easiest way to Heathrow from King's Cross/St. Pancras as it's just one train which goes direct, but it is not the fastest, as the train stops many times along the way.

(To others reading this answer: for some journeys there's yet another alternative of a RailAir Coach Link ticket, but this does not apply for journeys from Attleborough as far as I can tell).