This actually reflects reality from years past. When I moved to Staines in 1973, the county of Middlesex had already been eliminated, but still existed as a Postal District so I had a Middlesex address.
On examining my house-purchase document however it stated "...Staines, in the County of Surrey". But I found Surrey residents were reluctant to accept this geographical reference, sticking to the now defunct Middlesex (they tended to look down on Staines, now of course known as Staines-upon-Thames so probably acceptable these days, who knows).
P.S. Apart from airport noise, I found Staines a very nice place to live in those days, I don't know what it's like now. My move to Hampshire in 1988 was work-driven after redundancy.