If you have an invitation to watch cricket or the regatta wear your tweed sport coat or blue blazer. 
NOT TRUE! You can wear anything text taken from and copyright of projcetbritain.com

An invitation to someone's home is more common in England than in the rest of Europe. 
I wouldn't know. We do visit friends and family though.

Do not discuss business at dinner in someone's home unless the host initiates the conversation. 
True. They will think you are boring if you do. It is best to keep work matters at work

In England arrive at least 10-20 minutes after the stated time for social events at home. In Scotland and Wales, arrive on time.
I would arrive on time. If my guest arrived early I might not be ready.

Host or hostess always initiates first toast, which is usually only given at a formal dinner.
Toasts (when glasses are raised and some makes a short speech) are usually only given at special occasions like formal dinners, birthdays and weddings. text taken from and copyright of projcetbritain.com

At a formal dinner, do not smoke until after the toast to the Queen or until otherwise indicated by the host.
Guests at a formal dinner may not light a cigarette until after the Queens Toast or until the host gives permission. At a very formal dinner you should not leave the table until after the toast.

When finished eating, place knife and fork side by side on the plate at the 5:25 position.
Wrong They should be placed at 12 o clock (12:00)

You should leave a very small amount of food on your plate when finished eating. 
Not necessarily true but if you do it means that you have had enough to eat. Leave too much and your host will think you hated their food. text taken from and copyright of projcetbritain.com

The guest of honour should initiate leaving a party.
Can be true but if the party went on too long I would make an excuse and leave.

When the host folds his napkin, this signals that the meal is over.
Never heard of this one. If the host folds his napkin and puts it on the table it means he has finished eating. Other people may still be eating.

Leave a dinner party shortly after dinner ends. 
Not necessarily true, depends on the host. They may want to chat more.

Write a thank you note to the hostess.
Yes, that is always a lovely thing to do

If you go to someone's house for a meal, take a bottle of wine or some flowers for the lady of the house