Dillon is wearing a white Italia '90 t-shirt, but when he goes to snort his coke he is wearing a gray Sergio t-shirt.
The BT logo on the telephone box Matt enters is incorrect for 1990. The 'Piper' logo was not introduced until 1991.
The 10 pound notes and 20 pound notes used in the film are the "series E (variant)/Series V" notes - with the large digits in the top left corner - that were in use around when the film was made. They had not yet been introduced in 1990 when series D notes would have been used. Original series E notes followed in the early to mid 1990s but the Variant notes seen didn't appear until 1999 (£20) and 2000 (£10).
Much of the music used in the film doesn't fit the timeline. Prominently featured early is Ken Doh - Nakasaki which wasn't released until 1996.
Inner City - Pennies from Heaven was released in 1992. Also during the "fag" machine theft scene as the boys run out a N registered VW Golf can be seen.... N plates started around 1995.