I think you will get a concensus on this forum that a window is a good idea. That is probably the end of the concensus. You will get a lot of different ideas on where to put it.
To me, there are a few good reasons to have a window. You can see in there without artificial light. There will be times you are in there without the door being open. Being able to see is nice.
They will be able to see to eat, drink, find nest boxes, whatever. A big plus is the problem of them going to bed. They do not see real well in the dark. They are not totally blind in low light, but they do need a minimum level to be able to see. If it is fairly light outside and they decide to go to bed, the coop may be too dark for them to want to go in there, let alone them be able to find the roosts. Having a window to provide natural light is a big help.
Which side to put it on is a difficult question. You will find people passionate about the south side, west side, east side, or even a few that favor the north side. I have mine on the north side, not that I am passionate about that. It was the only outside wall of my coop I built so I put the window there. It works.
Some things to possibly consider.
1. Do you have a light outside that will be on all night? I'd avoid having a window that let that light in so they can get some down time in the dark.
2. I know you are in England, but I'd still consider the greenhouse effect. Will the sunshining in oveheat it? Maybe if it is a small coop and they will spend a lot of time in there during the day, but with adequate ventilation, it should cool off plenty by nighttime. On the other hand, will the sun shining in give it needed warmth. I lived in London for about a year and do not think either is a big consideration for you, but something to maybe consider. Your temperatrues are just not that extreme.
3. Some people like a bright coop. It seems more cheery to them. I think chickens prefer it a bit darker. It seems to have a soothing calming effect on them. In truth, it is probably not a critical point, more your personal preference than anything else.
4. I notice mine seem to like to roost in front of my window as opposed to further away. I'm not sure if it is because of the window or something else.
5. Maybe put it on the western side so they can have more light in the coop to go to bed by.
I'm really having trouble coming up with criteria that makes a real difference in my mind, maybe 1 and 5 with the rest not all that significant. It probably really doesn't matter where you put it as much as that you put one.