To respond to your question- security is in the middle of the terminal:
there is a First Class section on the left side (but BA has no first class on LAS-LHR.)
there is a TSA Pre-check on the right side (assume you are not enrolled).

Lines can be quite long but T3 better than T1 and moves at a reasonable pace.
I get emails telling me to get there early many times with different airlines. IMO a CYA.

If you are business class can use The Club at LAS, if you have an AMEX Platinum or Centurion card you can use AMEX lounge.