£450 for six units is fairly standard for private candidacy, and is actually pretty good if it includes coursework modules.
The only time you'll ever get cheaper than that is if you find a school that a) accepts private candidates and b) does all the exact same exam boards and papers you want to do. Which is fine if you are self studying from AS as you can just choose their boards, but from A2 not so much.
In my search for an affordable exam centre I found lots of schools that charged virtually nothing, but just wouldn't have the same exam boards as me. I'm not 100% confident you can sit different subjects at multiple places either.