Easiest from you is Milton Keynes - Olympic size, think they have some decent coaches, ice has always been decent when I've been there. I've always found it a pleasant experience. It's right by the station - few minutes walk - so easy if you get the train which is direct line from Berkhamsted. You could also drive - there's reasonably priced parking and I think you can park at Asda for free if you spend enough, which is right next to the rink.
Looking at the map the drive to Slough looks about the same, but I know nothing about the place and it's not really feasible by train. Oxford looks a bit further but also probably driveable, again no idea about that. Last one that seems practical would be Alexandra Palace - again plenty of parking. Train would be a little more awkward as you'd need to do Euston then tube and bus or train.
I've not heard great things about the coaching at Hemel.
Hope you find something that works for you. I'm in Hertfordshire though much further east, have similar journey times to what you are looking at, so tend to skate more often on wheels than ice as there are more options for that.
