I will be travelling from Cabridge, UK to London City Airport to catch a plane. Officially, the train ride takes ~1h45 (Cambridge-Kings Cross, then change onto the DLR).
How reliable is that estimation, i.e. should I plan half an hour extra (or more) in case there are delays? Also, how important is it to buy a ticket early - do I risk not getting a ticket at the counter when I want to leave around 5pm?
London City Airport doesn't have a railway station. It is on the Docklands Light Railway in Zone 3, on the Woolwich Arsenal branch.
Starting from Cambridge, you can buy a through ticket. You'll want either a single or a return, to London Zone 3. If you're coming back, you'll need to buy a return in Cambridge, as there are no National Rail ticket facilities at London City Airport. If you were flying into City and going to Cambridge, you'd have to buy a Zone 3 to Zone 1 ticket from the TFL ticket machines there (or use Oyster), then buy a train ticket at Kings Cross. Coming from Cambridge, you can buy a through ticket including tube+DLR.
Using a National Rail powered journey planner for the route (tweak the day as needed), we see that the predicted time is about 1 hour 45. However, the tube+DLR times from National Rail can be a bit vague. What you'd really need to do is use the TFL journey planner to work back to find your required arrival time into Kings Cross, then pick a train based on that.
TFL seem to suggest it'll take about 35 - 40 minutes travel time from Kings Cross to London City Airport by Tube (Northern Line to Bank) then DLR (direct Woolwich Arsenal service). However, that's platform to platform, so add an extra 10 minutes at Kings Cross to get from your train onto the underground, and then 3-5 minutes for the walk from the DLR station to checkin. Yup, that's right - with a brisk walk you can be at the checkin desks in 3 minutes from when the DLR pulls in!
Once you're at LCY, there is free wifi, a few cafes before security, and a few more after, so you should be fine if you get there early.
As for buying your ticket, it might be simplest to buy it online in advance. When you get to the station, you'd then just need to pop in your credit card, type in the booking code, collect your tickets and go. That's likely to be simpler than trying to work out how to get the ticket machine to sell a ticket to London Zone 3, which isn't always obvious... Assuming you don't have a railcard, it's £25.20 for an anytime single ticket (valid on all trains any time of the day), or £39.90 for an Off Peak Return (not during rush hours, out and back within a month), or £50.40 for an Anytime Return (any trains, return in a month)
There is no train station at LCY, the trip would involve taking the docklands light rail (DLR). I am not sure if there is a combined train/DLR ticket. If not you also need time to buy a DLR ticket.
I once took the wrong DLR line, although the lines are well indicated, but it then added quite some weight to the stress factor. Personally I would add at least an hour to your estimate.
If you arrive in time at LCY, don't worry about being bored. There is free internet and LCY is realy fun to spent time.
When driving fits the equation, going by airport shuttle might be a convenient option. When I lived in Cambridge I picked up friends from LCY on multiple occations. LCY is quite conveniently located even during rush hours it only takes a bit over an hour to get there. If I remember correctly, a prebooked airport taxi was between 60 and 80 £
Get the 010 Bus from Parkside to Bow Church DLR, then the DLR to LCY. Much cheaper and easier!
Regarding tickets I would try to get the ticket in advance. Cambridge station has very little "circulation space" outside the ticket barriers and in rush hour it tends to get packed with people.
