Your STCW does count but of course you have no loading/cargo ticket. I looked at transfering last year before I left the MOB but was told that I would have to go back to college to get a loading ticket and then spend around 6 months as a cadet on peanuts before being fully qualified.
You can transfer your qualifications by going through a merchant company, or through an agent like clyde marine (http://www.clydemarinetraining.com) who did 51% of all merchant cadetships last year. You wont get paid much (Â£120/wk at sea, Â£160/wk at college, Â£180/wk in exam phase) in this time but it will only be for a short time (about 4-6 months I presume? They are getting back to me with a definate timescale / job offer!) If you look at page 12 of DCI RN 2/2003 you will see the full details. Basically you actually get time at home when you work in the merchant navy, you get a normal life, time off, good money, you don't work for a company that demands more and gives less, you dont work for a company that is constantly shrinking, you can join a union who will look after your rights, and finally - you dont get your life screwed up by JPA. Oh, and you can just say "no, I am not doing it." and not get a courts martial.
I was initially put off by the low training pay, but when you see what a 2nd officer or chief officer gets paid (tax free too) then you would say its worth the investment. I am in the process of saving up to leave (what a predicament). On the plus side, at my interview this morning I was told of the working arrangements of some ferry officers - CalMac 3rd/2nd off - 2 weeks on then 2 weeks off PLUS 6 weeks annual leave. CalMac Mate / master - 2 weeks on 4 week off PLUS 6 weeks leave. If you have CQ1 then you are much better placed to get a Mates ticket (II/2) or masters ticket (II/2) after 9/15 months sea time (for respective tickets) and a few exams / courses.
See the below link for the syllabus for deck officers - even the masters stuff looks like pre-FNO stuff (standfast the cargo issues). Nice and easy.
Even although there is a DCI on this subject, the Clyde group have never encountered it before - I was their first case of an RN officer wanting to go merch. Basically the DCI just says that you need to have the exams, and the syllabus is pretty basic so I would assume that you can just sit the exams (with a 3 month stint of sea time). I got the pay (or allowance as they call it) figures directly from Clyde marine when I submitted my application. Best thing to do if you are interested is to fill out a form and go for an interview. My interview was really more of a chat than anything else and we both left having gained some knowledge - its your best bet if you want a hand. If you wanna use ELC then try going direct to warsash college as they do a bespoke course for RN - merch transfer, although I couldnt see if this was at all sponsored.
Since you don't appear to have actually even applied for an AIB yet, and you're thinking about leaving at the end of an Initial Comission you're trying to project in the order of thirteen to fourteen years into the future.
It's difficult to tell, right now, what things will have evolved like by that time. There is an increasing level of civilianisation of qualifications, and I would only anticipate that increasing over time.
Would you be able to go directly into the Merchant Navy?