Hi Meg - welcome to TE!
Our regular "greeters" must be out riding! If you have a chance, drop in on the Getting to Know You thread and say Hi.
Speaking for myself, cycling has been a huge benefit for weight loss (not to mention enormously fun - there's probably some sort of corelation there!), and I was surprised at how quickly I saw improvement. I've been riding a little over a year, and have lost more than 50 pounds (lets see - that probably about 3.5 stone, right?) and I still have something like that amount still to go, but it came off very easily (compared to decades of struggling with crazy diets and un-fun exercise). And I know the actual number of pounds lost doesn't reflect the fact that I've put on muscle and lost inches. I think you'll be very pleased with your progress.
Half hour out for your first ride is great! You deserve to feel chuffed! You'll be out riding longer and longer before you know it! You'll find lots of great support and advice here, and maybe even a riding buddy or two (we seem to have a growing contingent from the UK...)
I do recommend investing in a pair of padded cycling shorts once you start spending more time in the saddle - it makes a world of difference.
Welcome, and happy riding!