You'd be hard pushed to join the Merch as a Seaman being from the UK. Most companies use cheap labour from India, Philippines, etc.
A lot of companies do like the UK officers though. You can apply to a company - Shell, Maersk, Bibby, etc., and if successful they will sponsor you through a 3 year degree course with some seatime included. Once qualified they will normally take you on.
The application stage is carried out every September and January. Normally you'd go to the company offices and have a group interview with other candidates. That's if you pass the application sift. If successful you undergo an ENG1 medical with a specific GP then you're in.
The RN is very different. Obviously you can join as a rating from the British Commonwealth, do a basic written test at a local careers office, interview and fitness test, medical and security clearance and you're in. Can take upto 12 months depending on branch being applied for.
No general merchant navy.....Trinity House just advertised for rating apprentice