I was thinking the same thing, in international waters why can't any country just go in guns blazing and utterly destroy the pirates, imagine if a country actually sent a naval task force down there and just started laying waste to the pirate boats.
The problem is, the pirates tend to sit inside their countries territorial waters when they know a naval presence is around, and a full naval task force is massively expensive to waste on such a job given it'll only have an affect whilst they are actually there.
Much better to have some old fashioned fast response boats, with helicopters and marines spread out around the area (relatively cheap to keep in the area long term, much easier to find the manpower for, and doesn't detract from being able to send a full force elsewhere).
IIRC at the moment the RN doesn't have any vessels that are really suitable for dedicated anti-piracy roles, as they aren't very effective in larger conflicts so have been retired and not replaced over the years.
Of course, any serious response is going to cost money, for the boats, helicopters (remember the pirates tend to be in small fast craft so you need to be able to respond very fast), and crews - which would require a dedicated budget from the RN etc, and that would have to come from somewhere (the shipping companies would be the obvious source, but it's probably cheaper for them to pay the ransoms).