Prepositions at the end of clauses - perfectly acceptable in informal British English
Q1. That is the person I was talking ___
Stranded prepositions are perfectly acceptable in informal English. The prescriptivists can wait in the corridor.
Q2. What are you looking ___
Ending with a preposition is natural in spoken English. Churchill would approve. Or possibly not.
Q3. This is the book I told you ___
Stranded prepositions occur naturally in relative clauses. Grammar survives. Flexibility wins.
Q4. She is someone I can rely ___
Rely on becomes rely...on when stranded. Reliability is confirmed. Grammar adapts.
Q5. Which sentence is more natural in informal British English?
Stranded prepositions are natural in speech. 'With whom' is formal. Both are correct. Context determines.