Very good points from GW, spot on.
What an overseas applicant can do is contact the nearest AFCO to the sponsor's address in UK (ie the person/people you will be staying with when you come over). The RN Website lists all the AFCO's together with email addresses. If the AFCO has sufficient notice they may be able to book you in for Recruiting Test, Interview, Eye Test, Medical & Pre Joining Fitness Test in the one day, followed by booking you in for PRMC the following week, well in advance of your arrival. You would be advised to bring a copy of your full medical records with you, in case any medical issues need to be investigated.
The thing to remember is that Security Clearance can take quite a while for overseas applicants, so if you contact the AFCO & get the ball rolling before you arrive, so much the better.
From passing PRMC to being allocated an entry date is usually 6 to 8 weeks, so you need to consider where you wish to be during that period.
It also goes without saying that you mustn't travel halfway around the world at your own expense unless you are 100% ready to attend PRMC with regard fitness.
Best of luck.