Do I always have security of tenure?
No. Some leases will specifically state that the tenant does not have the protection of the Landlord and Tenant Act 1954, which normally provides for security of tenure. As a general rule, you should be very wary of giving up the security of tenure that the law normally provides. However, there will be some cases where a landlord will not be prepared to sign a lease except with this exclusion. Even where you do have security of tenure, that shouldn't be taken as gospel as a landlord might wish to redevelop or sell the property at the end of your lease period, for example.