What does it cost (UK £) to learn scuba diving and get all the equipment? Is it an affordable hobby? Good hobby to get into?

I am not familiar with the currency rate for UK, but I can give some ideas in dollars.
First, you need to get certified. This usually runs between $200 – $300 dollars, and should include your own snorkeling gear. After this, you’ll know whether Scuba is for you.
Equipment varies greatly in price, but decent middle of the road (safe) gear should run near $1000 for regulator, BCD, weight set-up, guages, etc. From there you may decide to upgrade some more. (the sky is the limit.)
As for being a worthwhile hobby, this is my opinion. IT’S AWESOME! There is more beauty underwater than I’ve ever seen on the land.
The general rule for exploring reefs = Take Pictures, Leave Bubbles.
Good Luck, and Safe Diving!