Indeed Hemel is closest though it's not full size and has pillars in the middle.
You could try Peterborough or Cambridge, both full size and about 30 mins on the train from Hitchin.
Or you could travel down to Finsbury Park, again about 30 mins on the train, and then up to Alexandra Palace by bus or train.
If you can drive, Milton Keynes is a bit further than Hemel but again full size, more choice of coaches undoubtedly, or even Lee Valley.
There's free parking at Alexandra Palace and Lee Valley.
There's a roller rink in Welwyn Garden City.