In general, the UK (unlike some other European countries) imposes no restrictions upon minors leaving the country either on their own or accompanied by an adult.
I've been abroad many times with a family friend - he's now 20 - on day trips since he was 10 and never been stopped by the 'authorities'. (Although these were day trips, the officials at the sea ports wouldn't have known that we weren't planning to spend longer abroad).
Back in the late 1980s, I took my former girlfriend's 12 year old son on holiday with me (principally so that she could have some time alone with her new boyfriend). I got quizzed by officials at the sea port but they still allowed us through. The following year I took the same lad abroad by air. This time I made sure that I'd got a signed document from his mother before travelling but we were never challenged at the airport.
So there are no no general rules which prevent you from taking any child abroad. There may, however, be conditions imposed by any 'parental rights agreement' that may exist between your wife and her former partner. This will be specific to your individual situation. You need to ask your wife whether there are any specific legal restrictions which relate to your step-daughter travelling abroad. If so, it may be necessary to seek the written approval of the girl's father.
Chris