Read that site thoroughly and consider what they don't tell you (or what you have to go hunting around the site for). They may well provide what they say they will for the prices they quote. However, in line with most correspondence colleges I've come across, their A level courses include only tuition and support. You'd have to get an independent marker for any coursework and you'd also have to make your own examination arrangements. Both of these will involve external fees.
They claim their diploma courses are 'level 3' - but level 3 what? It doesn't say. You are left to assume. Neither do they say anywhere, as far as I can see, whether their diplomas are accredited by any universities or appropriate professional governing bodies. You may well learn plenty, but I doubt the actual diploma certificate is worth diddly squat to any employer worth their salt.