How take a rail journey from London to Cambridge

Final Destination: Ely or King's Lynn

At King's Cross station take care that you will boarding the trains which make their first station call in Cambridge; these are the trains which will be usually heading beyond Cambridge to terminate in Ely or King's Lynn.
(Thameslink also operates 1 or 2 slower trains per hour, which complete the journey in around 1hr 25mins, so they are best avoided.
But those slower trains usually terminate at Cambridge, so they will stand out on the departure screens).

Note that this route is (was) is very popular with commuters, and as seats can't be reserved on these trains, try to avoid departing from London between 16:30 and 19:00.

Book early and save: No, you will pay the same price if you book tickets last-minute at the station.

On this London to Cambridge route operated by Great Northern you won't save money by booking in advance online, but the price you pay will depend on when you will be travelling
You will save money if you make a day trip by train from London to Cambridge on this route at a weekend, or on a national holiday.

The £17.50 is the 'Off-Peak' price for a single ticket, but Great Northern doesn't seemingly offer Off-Peak single tickets on Mondays to Fridays (except for national holidays).
So if you want to book a single ticket and travel on Mondays to Fridays, when they are working days, you will have to book the more expensive 'Anytime' tickets, no matter which departure you wish to travel by.

But if you will making a day trip to Cambridge by train, you can save money by booking Off-Peak Day Return tickets.
On weekends and national holidays, the price of a day return ticket is only around a £1 more expensive than an Off-Peak single ticket.
Similarly an Off-Peak day return ticket on Mondays to Fridays is only around £1 more expensive than an Anytime single ticket.

However, if you can book ahead and don't mind being committed to taking specific trains, you can travel each way for £8,. if you purchase the cheapest advance tickets on the alternative Greater Anglia services.
So on Mondays to Fridays you can save around £8 when making a day trip between London and Cambridge if you take that route.

Final Destination: Cambridge

**These trains also call at: Blackfriars and City Thameslink and Farringdon and St Pancras International

If your train journey to Cambridge will be commencing near London Bridge, Blackfriars*, City Thameslink or Farringdon stations than taking these direct Thameslink trains will save you the bother of heading to King's Cross and making connections there.
*Blackfriars station now has an entrance on the south bank of the River Thames, conveniently located for the Tate Modern and the South Bank arts complex.

Note that this route is route is (was) very popular with commuters, and as seats can't be reserved on these trains, try to avoid departing from London between 16:30 and 19:00.

Book early and save: No - you will pay the same price if you book tickets last-minute at the station.

The £17.70 price applies if you board at St Pancras, tickets cost around £1-2 more if you board at London Bridge, Blackfriars, City Thameslink or Farringdon
Note that booking a direct train ticket from those four stations will be cheaper than taking the Underground to King's Cross station and then booking tickets there for the alternative, faster London to Cambridge trains.

On this London to Cambridge route operated by Thameslink you won't save money by booking in advance online, but the price you pay will depend on when you will be travelling
You will save money if you make a day trip by train from a station in central London to Cambridge at a weekend, or on a national holiday.
On weekends and national holidays, the price of a day return ticket is only around a £1 more expensive than an Off-Peak single ticket.

Similarly an Off-Peak day return ticket on Mondays to Fridays is only around £1 more expensive than an Anytime single ticket.

Final Destination: Cambridge or Cambridge North

All trains also call at: Audley End

Aside from at evening business hours on Mon-Friday, in each hour one train makes fewer station calls than the other, so is around 12 minutes faster.

Not every departure will be by the newer Electrostar trains.
Some older trains which don't have Wi-fi and wheelchair facilities are also used on this route.

Aside from the price*, taking these Greater Anglia services are the best option for a train journey from London to Cambridge, if Liverpool Street station is convenient for the starting point of your end-2-end journey.
*if you want to purchase single tickets in particular

Book early and save: Yes

Tickets are usually placed on sale: From 12 weeks ahead of the travel date.

Greater Anglia operates these trains from Liverpool Street to Cambridge and it offers discounted 'Advance' tickets on this route.
They are available in limited numbers on most departures, but you need to book them online at least a day before you'll be travelling.
However, to obtain the £8 price, you will usually need to book at least a couple of days in advance and avoid departures at the busiest times.

The two disadvantages of booking an 'Advance' ticket is that they can only be used on the specific departure you select when making a booking - and then if you don't take that train, because you change your travel plans or don't get to the station on time, they can't be refunded.

If you can track down those most heavily discounted tickets then they're the cheapest option for a London to Cambridge train journey, neither Great Northern or Thameslink offer Advance tickets on the routes which they operate.