You don't mention your age, schooling background etc. But in taking it read, that you are Medically & Scholastically in the ball park. You will have to aim directly at the goal:- that is being a Helicopter pilot. Target the Service that gives you the best sporting chance of attaining that goal. All the Armed Forces can get you there, and with excellent training.
But the Royal Navy is a primary Direct route. You have a much greater chance of applying as a Direct entrant Aircrew candidate and if successful in passing the pilot aptitude test, medical and Interview board, you will be in to train as a pilot, 90% as a Helicopter pilot, where you end up is your choice.
The RAF have much the same criteria, but they want Fast Jet pilots first and aim to fill that area first with Multi/Rotary second.
The Army has very limited Direct entrant Officer intake into the Air Corp. (The NCO route is much too hit and miss, but if all else fails it is a worthy undertaking.). Joining Sandhurst as an Officer in another Corp apart from the AAC, will not guarantee that you will make the grade as a pilot especially as the stricter Medical/Aptitude criteria await your down the line and if you fail that you will need a quick career choice to get back to Aviation.
The Royal Marines = Same as the Army only they are tougher.
Good luck, whichever Service route you take you will have a great experience.(The private option is an expensive option, unless you gain sponsership). Keep the goal in sight, don't be talked into taking anything else.
Flown Navy
Dug Army
Eaten Crabette