Hi there!
I went to Herts Open Day about a month ago and I found the train journey totally manageable. Not only is it a fairly regular train, but some of the countryside around Hatfield is beautiful too. It takes about the same amount of time on the way back. The bus connection from the train station was pretty ace aswell (not sure if it was just because of the open day so hopefully somebody can clarify that for you).
If you've already got accomodation in London it might be quite valuable to make the journey to university. Although halls are great for the first year, it can get fairly expensive- I know the accomodation on Herts' campus aren't the cheapest.
Good luck with making your choices!