I’m don’t think the status of his visa, if it was indeed out of date, is relevant other than being a reason he might run if challenged. The cops didn’t know who he was, let alone the status of his visa.
Well he probably didn't know who they were. They were in plain clothes - he could have thought they were a gang - especially if they were young/looked young.
How is it not relevant?
OK had he renewed his visa, he would be dead with a visa.
But had his visa expired and was unable to be renewed he wouldn't have been there.
Especially with things like this going on:
Raids smash student visa racket
The raids were carried out by the Operation Maxim team
Twenty people have been arrested in a series of dawn raids to crack down on a major student visa immigration scam.
More than 120 officers from Scotland Yard's specialist crime directorate searched 12 addresses across London and Essex.
These included two suspected bogus colleges in Tooting in south London.
The raids aim to end an immigration racket estimated to have brought about 1,000 people into London and earned the perpetrators millions of pounds.
Wednesday morning's operation, codenamed Taming, was the largest yet to be carried out by the Metropolitan Police under the banner of Operation Maxim, an initiative to tackle organised immigration crime in London.
END
You may say, "All the more reason to run"
You also have to remember: The police saw him come out of the apartment they had been watching (not the flats in general), saw him get on the bus (?) and followed him all the way to the stop where he headed for the tube station. Does that seem random to anyone?