Apart from which dictionary to use there is one other difference between Scrabble play in North America and the UK. In North America, if you challenge an opponent's word and it is subsequently found to be in the dictionary then you lose your turn. In the UK a challenge may be made without penalty.

Which is the better rule rather depends on which side of the pond you live. Proponents of the penalty challenge argue that if you know a word and your opponent doesn't, then this rule rewards that extra knowledge. In addition, in tournament games which are timed, usually 25 minutes per player, then, if there is no penalty, unscrupulous players might challenge to get themselves more time, whilst the word is being checked.

The main objection raised to the penalty challenge is that it places too high a premium on word knowledge at the expense of other aspects of the game like strategy, anagramming ability etc. Opponents of the rule also point to the element of bluff which the rule allows - if you know that you have a better word knowledge than your opponent and he knows that as well, then you may well play a plausible word guessing that he is unlikely to challenge when he could lose his turn by doing so?

The last year or so has seen a compromise rule introduced in some international events. Each incorrect challenge receives a penalty of 5 points. This is enough of a penalty to stop frivolous challenges to gain time but is not as draconian as losing a whole turn.

A number of other variants of the rule exist around the world and the penalty rule can be different from country to country.