SMD,
The first and most obvious route is the Defence Techinal Undergraduate Scheme. The scheme is described at this link, with some details (such as bursary),:
DESG DTUS
It is rather better described here:
Welbeck DTUS Page
Data on In-Service degrees is less clearly defined, although it could be done better. But essentially In-Service Degrees are open to technical officers in technical subjects. The technical corps are RE, REME, RLC, R SIGNALS. These usually take place after your first appointment.
All officers can gain the following qualifications as part of their career progression:
Accreditation Page on Army Website
Plymouth/Portsmouth University through Wilts & Somerset Colleges Partnership - progressive route through from Level 5 to an MA or MSc with E-learning Foundation Degree in Business and Management
Bournemouth University - e-learning pathway to a full BA (Hons) in International Business & Management
Newcastle Business School/Northumbria University - BA (Hons) Leadership and Management Work based Learning Award
In addition the Army trialled a non-technical BA/BSc from the OU in 09-10, but the results have not been made available, as far as I know, in relation to take up and success.
You may have heard stories from the past about In-Service degrees in non-technical subjects: they no longer exist.
Later in your career you may become eligible for courses at the Defence Academy which will either go towards, or complete the necessary for Master's level qualifications (such as the long (course) information systems MSc). Likewise the advanced and higher courses of the staff college could get you a further Master's qualification including the MDA.
The Army has access to MPhil courses at Cambridge and other universities.
If you go on some overseas staff college courses you may get a degree from them too.
The Modular Masters Programme has been terminated except for those already enlisted on courses.
There are options through the enhanced learning credits system to get a degree partially funded by the MOD.