The easiest way to watch NHL free of charge in the UK is to take advantage from a betting site's live streams. Several bookies, including Bet365 and Unibet, allows their customer to watch the NHL without a paid subscription.
Opening a live stream is usually done in the following way:
Go to a betting site that offers NHL live streams
Create a new account or login
Choose "live streams"
Bet and watch your preferred hockey game!