I was confused by the £ by the lay odds too! But it relates to the size of your lay bet, not the size of the liability.
Also don't necessarily be put off if there isn't enough money available in the exchange at that moment in time - if there's still hours before kick off (or horse race, etc) there's plenty of time for your bet to be matched.
Only exception is with Smarkets who will leave your bet unmatched IF the odds go up and don't return to the odds you placed your lay bet at, iyswim. Betfair will match your bet as soon as money becomes available regardless of odds changing (except for in play bets, but don't worry about those yet
)
You'll learn as you go along, I promise!
I'm a fan of Oddsmonkey, personally. It's £15 a month but I wouldn't be able to MB without it.