You've got a few options mate, but these are the relevant points:
1. If you wanted to study for an open university honours degree whilst in the forces it would take 6 yrs of part-time study.
2. If u went to Sandhurst minimum return of service is 3 and a half yrs, after which you couldn't 'transfer' to RM, you could leave and apply to join RM as other rank or YO depending on *text deleted* age.
3. You could go to Sandhurst and do the All Arms Commando course and get *text deleted* green lid that way, that would probably mean joining artillery or engineers.
4. If you joined RM, you would hav the same opportunities to study for a degree as if u went to Sandhurst, probably more so if u were a YO
5. Couple of months ago, I met a 20 yr old RM Lt, joined straight after school and was in the process of studying for an open university degree in Law, no doubt free of charge
In conclusion, if u want to be an officer go for RM YO 1st, Sandhurst as a back up. If u want to be a Marine go for RM YO 1st, Marine other rank 2nd. Most officer candidates have degrees so u may be at a disadvantage in terms of both academic acheivment and maturity. Also, there is the pay issue to consider (you will be paid more a graduate). But there is no point going Sandhurst just to get a degree, there are no more opportuinties to enhance *text deleted* education in the Army than there wud be in RM.
Young Officer-sorry, should have been clearer. YO's are basically the equivalent of Officer Cadets at Sandhurst. RM YO's are 2nd Lieutenants from day 1 of their training whereas at Sandhurst you would be an Officer Cadet until you pass out after which u are commissioned as a 2nd LT. Any questions ask away