Hi, Please can somebody order the longest to shortest Thames Valley branch lines? When lockdown is over I am planning to do the Thames Valley day ranger,so some background information would be very useful.

Take a look at this ...
What is the longest branch line in Cornwall?
Looking at the Adlestop map (
https://www.systemed.net/carto/New_Adlestrop_Railway_Atlas.pdf ) which is geographic and clear, the longest branch that remains open is Newquay. In the past, there may have been an argument for the Padstow
LSWR▸ /
SR▸ branch to be considered but I suspect that depends on whether you count it back Halwill Junction or to the Devon / Cornwall border.
Same map ... same question as how you define "branch" and "Thames Valley".