Hi,
On Rightmove and SpareRoom, they show ensuite rooms available which are less likely to be box rooms compared to if you chose to live in halls. My recommendation, would be to just check everyday because the website gets updated with new rooms very frequently.
Having said this, rooms are always going to be smaller and more expensive if they are ensuite. So, although I know you mentioned you want ensuite, you could look at houseshares as a backup as many houseshares do have multiple bathrooms so it is not bad sharing a bathroom with other people as the bathrooms do not tend to be busy. Houseshares are also alot cheaper.
I had the same dilemma as you for when I was moving into second year - I was first looking at ensuites and studios. But, I found, it was too expensive so I then opted for a houseshare. The house I live in has 2 showers and 3 toilets between 5 people and the bedroom is also larger than any of the ensuite rooms I've seen. And, there is a bathroom is right next to my room so it is convenient. Hence, I chose that and am opting to stay for another year.
Best Wishes,
Gulcin