I think everyone wants to know this before starting out don't they? How realistic is xyz per car? How many cars can I sell a month if I set my margins to xyz?
It's understandable but firstly, no one ever openly discusses margins in public for several reasons, most of them rather obvious. A less obvious one being that if you are pulling good money from nearly every car then people don't believe you. So it's best to find your own way, be happy with what you're getting and don't be bitter and jealous of those that are doing better than you and try to find ways of increasing your margins if you're not happy with what you're getting.
But £800 gross on a £6k car isn't particularly ambitious and to be honest, likely to lead to disappointment long term unless you're doing serious volume. Advertising, VAT, Tax etc will catch up with you and that's before you account for prep, warranty claims, the odd refund and some good old fashioned sticky cars that won't move no matter how many of their legs you chop off.