score:6
When your visa is issued, it will have a fixed start date. They will try to align the start date to the travel dates you gave in your application. This does not always happen, and if the start date they give is wrong for you then you can return your passport and request another start date.
For planning purposes: Visas can be forward dated up to 90 days from the application date. The application date is the date they collected the fee.
For your other question about arriving in the UK prior to your visa's start date... Hopefully the airline would prevent you from boarding, but in the event that you arrive in the UK outside of your visa dates, you would expect serious problems for abuse, up to and including removal. The situation is explicitly covered in Paragraph 30C of the Immigration Rules.
An Immigration Officer may cancel an entry clearance which is capable of having effect as leave to enter if the holder arrives in the United Kingdom before the day on which the entry clearance becomes effective or if the holder seeks to enter the United Kingdom for a purpose other than the purpose specified in the entry clearance.