I've never had high tea before but the website says it is $10 more than afternoon tea and served in three courses. It says it is more elegant and will give you the history of tea as well.
Olde English Tea Room - Special Occasions
I'm not sure high tea is available to individuals as it seems like it is just for groups of 20. Although it is possible they will make an exception.
Here is the menu for what is offered.
Olde English Tea Room - Menu