Club night is Monday. The rest of the week, these are the places worth your time.
Lichess
Lichess is free, has no adverts, and nothing is held back behind a subscription — the analysis, the puzzles, the studies and the tournaments are all simply there. If you only use one site, use this one. You do not need an account to play, though you will want one to keep your games.
Chess.com
Chess.com has the largest number of players and the most polished lessons. The free account is perfectly usable; the paid one mainly buys unlimited puzzles and deeper analysis.
ChessKid — for younger juniors
ChessKid is built for children, with moderated chat and a safer environment than the adult sites. It is what we would suggest for primary-age players rather than a general chess site.
Fritz Online
ChessBase offer a browser version of Fritz at fritz.chessbase.com: a willing opponent at any hour, with the strength adjustable to give you a real game. A free ChessBase account is needed to play, and it keeps a rating for you as you go.
A note on playing engines
Playing a computer is useful practice but it is not the same as playing a person. An engine set to your own strength plays like nothing you will meet across the board — it will not get tired, it will not go wrong under time pressure, and it will not be nervous. Use it to drill openings and endings, and get your real games against real people, ideally on a Monday in Watford.