If you leave the UK
before your application is decided, such application is considered automatically withdraw as per rules. So you cannot travel while you have a pending application. When your application is approved your old BRP is invalidated on the spot. The fact you receive a new BRP later does not mean your old BRP is still valid. You are admitted back at the border based on the conditions of your
current leave which are different for people with Tier 2 visa and ILR. Tier 2 holders are readmitted as visa holders, ILR holder are subjected to different rules as returning residents
Advice is given based on my personal research and experience only. Do NOT contact me via private message for immigration advice