Found this option hidden on the London Aquarium website. Regular price is £150 per person. Online price is £120 - 20% off at all the other websites .g. Thorpe Park, Alton Towers, Legoland, Merlin Annual Pass.com
HOWEVER, click through the London Aquarium website. You'll be put through to Maximweb. Click on a date to pick up, then scroll to the bottom for "Merlin Annual Pass Individual - Half Price! @ £75"
The website seems to fail with Firefox but worked with Chrome and IE for me. Picked up my passes today (the missus & me) - but you have to pick it up from the London Eye ticket office next door.
Also lets you skip the queues - which is awesome!