The two steaks that Roy and Vincent get from the Russian butcher shop are different at the shop than they are in Betty's kitchen. The ones in Betty's kitchen are thicker cut and narrower.
in 2009, Stephen tells Betty and Roy he is going to Spandau Prison to do research. Spandau Prison was demolished, literally ground to dust, and dumped at sea in 1987 to forestall neo-Nazis from turning it into a shrine. A shopping mall was built on the land.
The flowers in the painting that are referred to as lilies look like the flowers that are sometimes referred to as "calla lilies" or "callas". Those flowers don't actually belong to the lilies-family or Liliaceae, so they are a rather peculiar example of a flower called 'lily'...
While Roy and Betty are at Slough railway station, a train in a partly purple livery speeds through. This is one of the new Elizabeth line trains, which are still (late 2019) running experimentally and did not exist in 2009, when the film was set. Roy also leaves on a GWR branded train, in 2009 this would have had a First Great Western livery, as the GWR brand didn't return until 2015.
Although the film is set in 2009, Roy puts the current plastic UK £10 note on the counter in the butcher's shop instead of a paper one from the period.
Interior Roy's flat is a large room with a huge bay window with stained glass detail, yet seen from the street it is a modest size room with small, flat, sash windows. No attempt to match the interior to exterior.
The modern double-decker buses featured in some of the establishing shots of London were not around in 2009 when the film is set.