That's a good saving. Ours don't expire until August though, so hopefully they'll have some more offers on around that time.
We got 25% off ours, plus £250 of Lego vouchers too, so it was well worth it.
We went to the Sealife centre in Brum during the summer holidays. It was a rainy day and the queue was about 100 people deep. Merlin passes let us bypass the whole queue.

Same for all the parks too. You don't have to queue to get in. Shame you don't get to the front of the ride queue though...