Kapitain, your source has more info than I found. Here is the sight I looked at:
http://www.royal-navy.mod.uk/static/pages/2204.html
The 25 patrol craft on your list, I might find suspect as the site above lists 18 unarmed "Fast Training Boats". If so, I'd probably discount the "patrol craft" on your list bringing the number of vessels between the two nations closer to parity.
However, I do not mean to say that Japan's Navy is on parity with that of the Royal Navy, as you mentioned, Japan does not have even one aircraft carrier.
I couldn't find a good source on Japan's ships including their own website. Jane's is all paid subscription. CIA Factbook wasn't helpful. What I'm getting at is what was your source for the info? I'd like to be able to look at this info in more detail on other nation's navies.
Thanks
Kevo