What are the slash markings on Gatwick's 08R/26L?
They are &quot;Rapid Exit Taxiway Indicator Lights&quot; as described in section 9.6 of CAP 637.

9.6 Rapid Exit Taxiway Indicator Lights
9.6.1 Rapid  exit  taxiway  indicator  lights  (RETILs)  provide  pilots  with  distance  to  go information to the nearest rapid exit taxiway on the runway, to enhance situational awareness  in  low  visibility  conditions  and  enable  pilots  to  apply  braking  action  for more efficient roll-out and runway exit speeds.
9.6.2 RETILs consist of six yellow lights adjacent to the runway centreline and configured in a three/two/one pattern spaced 100 m apart; the single light is 100 m from the start of the turn for the rapid exit taxiway, see Figure 1.6.