If trains are missing in the rail planner app, this has no impact on the validity of your pass. In fact, the app is the last place to look for reliable timetable information, since it is offline and only gets an update about once per month. It's better to use national websites and apps, since these will be more complete and up-to-date. For international planning, use the DB website or DB Navigator app. It has about the same coverage as the rail planner app but has 2 updates/week.

For Great Britain, check timetables here.

Reservations in GB are optional (except for night trains) and can be booked free of charge at any GB ticket office or online using these steps.

YOu canNOT use it on the metro=underground in LON. There are however ThamesLInk trains running across town No-So from beside St. Pancras to Waterloo/London Bridge or aorund=change for Canterbury

There are also hi-speed trains to Cant from the €* stop using this line as far as Cant/Dover, and a pass should also be accepted-no idea how it works for these, REServ or whatever.

The local trains south of London (Southern or Southeastern) are very much like giant metroś for mass-transit and can still be overly overcrowded in the peaks for commutes.

You will likely have gates at all stops there-pass by a person who will wave you through.

Thank You- I have taken the train to Canterbury several times before, but never with eurail card. (family there)  

Just to confirm, can I use a Global Eurail pass to go to Leicester or Burnley or Leeds, or other stations missing from the Eurail pass map? Thanks to the experts for your help!

Yes, in UK the entire network is valid, exceptions as mentioned above; London Underground, DLR, Trams and Metros.

The Eurail maps, the PDF one and the interactive website one are only summary maps of what the map compiler considered main routes.

The map has no bearing on pass validity.

In some parts of Europe where there are few routes it may show most or all lines, in others with a dense network it omits the majority of routes and in certain countries with a propensity for suspending entire lines for years (France) it includes some routs that haven’t seen trains for decades.

Thank you!