Places are normally listed to rent when they are available, and people expect you to take them then (they won't hold it for months for you). Look about 1 month ahead. Also in the UK it is easy to move out, you have a set contract length of say a year or 6 months, then after that you can leave whenever you want with 1 months notice - so if the place you find initially isn't perfect, you are not stuck there for years.