Dolphins or whales? Or doesn't it matter? There are 20 odd species of dolphin / whale in the waters around the UK including Orca (the largest dolphin).
In NZ I've seen male sperm whales which was amazing but that frankly wasn't as good as watching dolphins in the UK. Try Scotland's resident dolphins in the Moray Firth, just stand on the beach and look as the tide turns, last time I was there a large male surfaced about 20 feet from me as I parked the car. Spent hours there watching a large group feed and teach very young ones how to behave while hunting. Just take binoculars or invest in a spotting scope. Also a trip to the Shannon Estuary in Eire is pretty much guaranteed to give you a large number to see.
Sadly a lot of whale watching trips are not organised as well as the one described earlier and many don't do no-see no-fee.