Indeed, drone technology is certainly - at the moment - in a more advantageous position on land. But that's not to say that deployment on surface ships would be useless.
I was thinking more along the lines of packing several Stingrays on a UAV, if there actually was a potential for ASW work, rather than going for the heavies as (at least, going by Wiki figures) 1 Spearfish for the RN equals 6 Stingrays so IMO the lighter variant would be the better way to go.
Hell, looking at the FireScout (again, Wiki figs), the MQ-8C has a payload of 600 - 700 lbs so it could just about be able to hold a single Stingray so at least currently, UAV tech isn't developed enough to be able to do anything effective in the ASW role.
Sorry I was not clear, I didn't mean an ASW UAV but an ASW USV, and much bigger than we have seen before, something that would compliment the Merlins and work with them. Something like an unmanned M80 Stiletto:
[ame="http://en.wikipedia.org/wiki/M80_Stiletto"]M80 Stiletto - Wikipedia, the free encyclopedia[/ame].
A Type 26 with 2 Merlin on board working as a mother ship, with 4-6 USV operating at say 100 miles radius, with their own dipping, even hull sonars (much bigger than possible with even a Merlin) all being coordinated by the ships TAS (almost an extension of it?). These USV could just drift to minimise background sound, and then use their high speed to relocate. Initially you could just fit helicopter detection kit, but there must be massive scope to improve on this, with the benefit of permanent hull/sea contact and less weight restrictions.
Missions can be in days/even weeks not hours with a helicopter, and for the cost they could be a huge force multiplier.
As I understand it an airborne ASW torpedo is almost fire and forget, whereas a Spearfish can be fired and then controlled by the launch platform, also a much faster and/or longer range weapon; never an option for aircraft, but on a USV potentially another real advance. The warhead is probably a bit of overkill, but they could always scale it back and replace with more fuel.