In the UK the balls are often red and yellow balls in replacement of the solid and striped balls in the US version although the black ball is always black (no kidding). One difference which can seriously affect play between the two countries is that in the UK the pockets are significantly tighter or smaller than in the US. The UK eight ball pockets are usually only fractionally larger than the ball and have rounded corners. For the US tables the pockets are quite a bit larger, therefore easier to pot and often the edges of the pockets have the angled sides which is more common in nine ball.
In the UK pool pub rules that are played frequently there are many changes in what constitutes a foul shot. If the player misses all the balls on the table then there is a two shot foul known as the "two shots" rule for the other player. It is even commonly played that if the UK pool player misses the black ball when they are on the black as their final ball then it is not loss of the game but is only a two shot fould for the opposing player. Many US pool players (and others from around the world) find this rule particularly hard to deal with as the rule in most other countries outside of the UK is that a miss of the black ball while on that final ball is loss of the game.
If players from differing countries are playing against one another, it is a good idea to clarify the rules being played first to avoid argument. This also goes for people who would like to play online pool however instead of discussing the variations of the rules with the opposing player it is the software which will decide therefore the player should read all of the rules available on the online pool website.