It currently seems that you're only able to pursue a degree as an officer in the British military through institutions such as the open university. However, I frequently see military officers' biographies that state they joined, and then attended university. Is this a simple change in policy? Or are there more detailed reasons?
Do you mean getting advanced degrees like Masters or PhD's? Or that they were commissioned and at some later point completed their bachelors.
As for the first option continuing education has long been part of many military careers, with the institutionalization of linking an education to the Officer Corps in most Western nations as education infrastructure advanced so too did that relationship.
To take the similar example of the US, long periods of peace, stable long term postings, and a desire to standout without combat experience can make the gaining of another degree attractive. And with the explosion in attendance of universities from the mid 1800's on, and the foundation of ROTC and many service schools streamlined the process. To the point that today any officer considering making a career of the service is essentially expected to both gain an advanced degree from some source(perhaps even on the side if attached to an ROTC unit) or attending some form of War/Staff College.