Complex sentence: Sinclair is interrogated by the gang's leader, Sol, who plans to use her as leverage to cross the wall, allowing his army to start their invasion on England; it's the only way to change the UK back to its formal glory.
Split into simpler sentences: Sinclair is interrogated by the gang's leader, Sol, who plans to use her as leverage to cross the wall and invade England. It's the only way to change the UK back to its formal glory.