How would I say £10.50?
The standard way of stating this would be: 十英镑五十便士. Of course, as you might have suspected, there are other informal/colloquial ways of saying it. Here are some that I personally would not find odd:


十镑五/十块五
十块五毛 (based on the fact the pound/penny uses the same decimalized system as renminbi)
十点五(英)镑 
十磅半/十块半


If anyone has any other suggestions, don't hesitate to leave a comment.