A signifcant minority, if not outright majority, have done double aware science rather than individual sciences, over the years. Many have done well on the A-level science courses and at degree level afterwards.
Deciding whether to take it or not because of it's perceived difficulty is generally a poor way to choose subjects. Are you interested in the subject matter generally, and are you considering purusing related areas of study or work after 6th form, are better questions to ask yourself when choosing a subject, and better reasons to pick one.
If you're interested in various bioscience related subjects (which is a fairly broad array randing from anthropology, to psychology, to biomolecular and biomedical sciences, to ecology and conservation sciences) it's a pretty reasonable choice. If you enjoy learning about the natural world, then of the 3 "core" sciences I would argue biology best captures the essence of what you may have been exposed to previously (e.g. through documentaries and museum visits etc).
Now, if you aren't planning to pursue this area, and are strongly considering studying say, economics, then it's not particularly useful in that sense. If you are still interested in the subject matter it's not a bad choice, but it's not something you should choose just because you think you should have "a science A-level".
Difficulty at A-level is virtually impossible to quantify because what may be difficult for some will be remarkably easy for others, either due to better preparation beforehand, more motivation to explore the subject beyond the standard curriculum, or various other reasons. For example, for many people Further Maths is the hardest A-level available, however for many mathematically oriented students who enjoy mathematics and plan to continue studying it, it's not particularly difficult and is just as routine as any other subject for them.
There is a signifcant amount of information to learn for the subject, and compared to e.g. physics and chemistry, there are fewer "process based" skills and techniques to be learned that are examined, and more raw information. If you find retaining large amounts of new information difficult, you may find some parts of the course challenging. However this may well be balanced out by e.g. experiencing the actual concepts through lab investigations.