Autotrader is the biggest site I think apart from ebay maybe, Trade sellers list the same cars on both but I have seen the same car listed on ebay at a higher price.
It’s easy to sign up to glass.co.uk for a valuation.
Put a honest description in the ad explaining it’s service history etc, imo it’s better to see some kind of human input in the text part of the ad than just a basic list of the spec(auto trader should automatically generate the spec list for you when you list the car).
And some good pics also.