Can you post things in a UPS envelope, via Royal Mail?

Selling off a load of old magazines, and have had someone who wishes to buy a group at a time... tried to fit them into a plain, blank envelope but didnt fit, however found some old UPS envelopes which they all fit in fine.

Can I package them up in this, put the address in, and send this via Royal Mail? Or wouldnt they accept it?

Thanks