Whether you are being unreasonable or not depends on information we don't yet have.
I can fully understand that your DH, having been given a cancer diagnosis, was very frightened and wanted to spend time with his family and have treatment at a place he feels more secure. And 4 months isn't a long time. So if the deal was, he makes a full recovery, then you move back to Bucks, in my opinion you are being unreasonable and should see it through with him.
If he wants to stay there after he has had treatment and makes a full recovery then that is a different scenario. He may well feel that way as a scare like that sometimes makes a person focus more on how precious time is with family. You have left your family and friends reluctantly because you had to consider his wishes as he is the one with the cancer. However, if you can see no return to the life you loved then of course, your health could suffer also and I have more understanding.
I think you need (if you haven’t already) to establish if this is forever. Then, decide if you want to bite the bullet and get out there and make friends, a new life, and maybe a move outside of the city when he doesn’t need to be so close to the hospital. You could travel down to see family on a regular basis. So stay there with him. Or, tell him that you didn’t see the arrangement as a permanent one and just wanted to support him during his difficult time but you want to return when he has recovered. Then I guess you have to decide what is more important to you.
If his prognosis is not good, then you must stay and support him through what will be very difficult times. You may find then that his family will be invaluable support to you.
Maggies, as suggested by petalmoore is an excellent cancer support facility and I would fully recommend it. It’s for sufferers and their families and it’s a truly wonderful place. I have experience of one in a different part of the country and can report that they are calm, serene and very spiritual places but they also hold classes for gentle exercise, cake making, music and much, much more. They have counsellors available if you are struggling with any aspect of how cancer is affecting your life. I would pop in there soonest and see if you can get some advice, support and you could even help them out if you want an activity. They usually have wonderful cake!!!! A good enough reason to visit anyway!