Gavin, it sounds like you probably did a "discover scuba" with an instructor. There's certainly no shortage of instructors in Cancun. Google for "cancun scuba". If you want to dive more often than the occasional (and expensive) discover scuba type deal, get hooked up with a local instructor. You could do classes in Cancun, or bookwork locally, and in water class in cancun, or do it all in cancun. Personally I'd do the class local, then do fun dives at the tropical vacation destination.
Just because you're certified, doesn't mean you have to dive with just you and a buddy. Operators will be happy to provide you with an in water dive guide for a nominal fee. It will likely be much less than what you'd pay for the "Discover Scuba" instructor.
As for the boat.. I'm pretty sure there are shore based dives in Cancun. That's where you gear up on the beach and swim on out yourself. If the boat is intimidating, this might be a good choice until you have more comfort in the water.
In general, the better trained you are the more comfortable in the water you will be. The more comfortable you are in the water, the more you can enjoy the experience. That's my opinion, anyway.
There are several dive shops that will teach you to dive in London. A quick google search shows that one of them is running a groupon for a 79 pound PADI class at the moment. You also have the famous "British Sub Aqua Club" in London. Usually the more expensive (and longer) classes are worth the extra time and money.
Good diving!