Factual error: When Bridget is walking to work (this occurs twice in the film, where I noticed the mistake) she goes through Piccadilly Circus. The next frames show her crossing Tower Bridge, as if it was just over the road. As anyone familiar with London will know, this could take the better part of an hour to walk this far.
Factual error: Bridget gets a screen notice ('One new email message') from MSN Hotmail. Hotmail does not give screen notices except for notices through MSN Messenger (which is not what Bridget gets).
Factual error: After Bridget arrives back in London from a visit to her parents, the camera pans away to reveal the train on which she is supposed to have travelled. You can tell from its distinctive yellow and white exterior that it's a Connex train, but these only run on the suburban commuter routes through south London to East Sussex, Surrey and Kent - nowhere near Mr and Mrs Jones's Cotswold home. Rail services from the Cotswolds are operated by First Great Western (grey high speed trains) and Thames Trains (navy blue exteriors) and terminate at Paddington in west London.
Factual error: One that only Londoners would notice or care about, but anyway: Bridget's flat is supposed to be in the trendy bit of Southwark, near Borough Market, i.e. south of the river. Right at the end, when she pursues Mark Darcy, the shop where she catches up with him is clearly in the City of London (the very distinctive Corporation of London bollards are all around, and the shop itself is on Threadneedle St, I think) - meaning that he's walked and she's run right across Southwark Bridge and up King William St in about thirty seconds...
Factual error: When Darcy leaves England and goes to New York City. He steps out of the terminal and a big banner reads Welcome to New York. Then he stops by a speed limit sign that said 15 miles per hour. Problem....that sign is red with white letters. The U.S. speed limit signs are not that color. Therefore, that airport was NOT in New York and they were saving money on production. I guess the banner helped.