This is not surprising, for two reasons:
First, back in the sixties and seventies, university students were idealists. They thought the purpose of going to university was not to get a job, but to expand their horizons, make them wiser, give them thoughts with which they might change the world, etc. Times were good then, governments were expanding higher education and providing generous grants and low-interest loans to students, governments were hiring people with general B.A.s as civil servants, etc. Students then assumed that walking into a good middle class job would be easy after going through a consciousness-raising B.A. in Political Science or Philosophy or Comparative Religion.
But times have changed now. University tuition fees and living expenses have skyrocketed, and in many countries government subsidies have not kept pace, so higher education now entails huge debt for most students. Further, most civil service hiring has been frozen, and the few civil service jobs that come open are almost always in something like accounting or computer programming or personnel management or the like. So one can no longer count on getting an Arts degree and walking into a lifelong civil service job. So students have become more calculating about their education. If they are going to accumulate tens of thousands of dollars of student debt, they want to be sure of a high-salary job when they graduate. So they now tend to major in Business or Computer Science or Engineering or one of the fields allied with the Health sciences. So it’s not just theology programs that are experiencing low enrollment. English, History, Philosophy, etc. are all taking a hit.
Second, in Britain church attendance is abysmal compared to that in North America. So there isn’t the base of kids coming from families where religion is an everyday topic of conversation in the UK that there is in the USA. Probably during WW II a high percentage of Britons went to church semi-regularly and thought of themselves as Christian. I don’t know what the current fraction of regular churchgoers in Britain is, but I’ve heard numbers in the range of 9 percent. It’s not likely that students brought up in completely secular homes are going to spontaneously decide to invest four years of their lives in the study of religion. It’s not even likely that they will take an elective course in religion, if religion has never been talked about in their home and none of their friends or neighbors are religious. They will feel no connection with the subject. For an elective, they will probably take sociology or psychology instead.