Under Rights of Light Act 1959. This gives a period of use which gives rise to easements.
For Rights of Light the period is 20 years to create the easement and only written permission will defeat it.
For example someone paying a nominal fee to an adjacent landowner for his light. Whether the fee is ever paid or not, this written permission means that it can be withdrawn by the landowner at any time and stops a right of light coming into being.