I went ice skating today, at Kew Gardens. Every year in winter they have this outdoor skating ring and it's great. The price is sort of reasonable: £10 for 1 hour, including rental skates.
I was a bit shaky on my feet at first because I hadn't skated for years. But then it's like cycling or skiing: once you're back on the ice, your body remembers. After 10 minutes I was ok and really enjoying it. However the weather is quite mild at the moment considering it's the end of December so the ice was quite wet and deteriorated very quickly. The rental skates were also quite blunt. This plus the fact that I'm a very average skater meant the skates were constantly sliding sideways so it was a bit more strenuous than it should have been.
All in all it was a great exercise and I really enjoyed it. I'll have to go try the rink at Somerset House to compare.