Return WP document to the emlpoyer?

Hi

I for my FLR on WP done at Croydonlst month. I took the Work Permit document with me. What they issued was a visa stamp on my passport and a letter stating for leave to remain.

So what I have is

1)Work Permit document which my emlpoyer gave me
2)Stamped Passport
3)Leave to remain document issued by HO.

My employer have asked me to give the Work Permit document back to them. Do I need to give them the WP document back or should I retain that?

Of course I will keep the Photocopy of it anyways.

Cheers,
Chris