Hostels and backpackers
Hostels or backpackers are attractive to newcomers to the UK, as they are cost-effective, allow you to meet other travellers and are not a long term commitment. A quick online search will bring up thousands of options for you to choose from.
Booking is not usually required, but be careful as they can become quite busy around peak seasons and weekends. Ensure that you inspect the room and facilities before checking in or making payment.