Have a look on the RYA website www.rya.org.uk
for details of distance learning courses.
Quite a few and all varying in price .
They do have their plus points in that you can study at your own pace but check out what type of student support they give in case you come across a problem that needs explaining in detail
The advantage of the class room is you have the instructor on hand to help you in those types of situations and you can work through your answers with them if you can't quite get it.
Good luck with it though .