Hello and thanks for using Just Answer.
My name is ***** ***** am happy to assist you with your enquiry.
In a terrace set up, it would be normal for one property to own the alleyway, subject to a right for the respective neighbours to have a right of way over it.
You can find out this information quite easily, by obtaining a copy of the Deeds from the Land Registry website-
The Deeds official name is "Official Copies" and here is the link to the Land Registry website-
You are probably best to get a copy of the neighbours deeds, and in the Register, it will say if the alleyway is subject to any rights of way in favour of your property.
If you do have a right of way, your neighbour is legally bound to keep free from obstruction the alleyway, to enable you to exercise your right day or night.
I hope this assists and answers your question.