When we see Dennis and Whit racing to the front of the Marathon, the shot cuts to Jake sitting on the sofa watching the television, and stands up when he sees Dennis and Whit, he is seen wearing a grey vest.
In the next shot of the same scene, where Libby has joined Jake, Jake is now wearing a khaki green/yellow t-shirt, with multi-colour skulls.
Dennis is arrested for buying tickets from a policeman posing as a tout (scalper) outside the theater showing "Lord of the Rings". In fact it is only an offense to sell tickets in this way, not buy them.
Dennis is incorrectly arrested for buying a theatre ticket from an undercover officer in a sting operation. It is not a criminal offence in England to tout genuine theatre tickets or to buy them from a tout. At worst it is a breach of civil contract with the theatre if they have inserted a "no sell on" clause into the terms of purchase, which would result in Dennis being refused entry by the theatre.
The fictional "Nike River Run" described in the movie as a 26.2 mile run along the River Thames with the finish line at St Pauls Cathedral. The race is shown as starting at City Hall on the South Bank of the Thames, which is only 1.3 miles away from St Pauls.