Well, if you set aside the obviously memorable Waterloo, Congratulations, Puppet On A String and Bomm Bang-a-Bang, i have forgotten most of them!
The ones that come to mind are All Kinds Of Everything (Dana for Ireland), Knock, Knock, Who's There? (Mary Hopkin) and Come What May (Viki Leandros).
My absolute worse Brisitsh entry must be Power to All Our Friends (Cliff Richard )