my stepson still uses his mothers maiden name, no problems at all, but then he was 17 when we applied for a visa for him, the thing is that's the name everyone knew him by and it's not a problem, but for younger ones it might be.
deed poll is cheap and easy, but if think you cant change the name on her birth cert, but if i remember correctly she can apply for a passport using your surname.
good his name is not on the birth cert, but you should speak to your stepdaughter and prepare her for an interview, just in case she is asked to attend one.