can be done instantly if a personal visit is made to a broker/insurer.
can be done over phone with some companies.
can be done via web with some companies.
unless there is some problem, such as the dwelling needs to be surveyed, usually only necesary if it is a period or high value dwelling or built of non standard construction.
If it is a simple pdh, build of concrete and roofed with slate/tiles and occupied by you and your family and used solely for residential purposeds, and that you have no coloured claims or convictions history, then there should be no delay
I recently secured next day cover online from 123. They followed up with a phone call the following morning and the paperwork arrived 2 days later. The premium was also less than half of my broker's renewal quote. So far I'm very impressed.