Why don’t British pubs/bars have poker machines?
I can't answer for British pubs because I am not a UK resident nor do I know their gambling laws but it's the laws governing gambling which dictate where it is and is not allowed. Where I live, gambling machines can only be placed in casinos. But you go some place like Las Vegas, they have slot machines everywhere ( at the airport terminals so you see them as soon as you get off the plane, gas stations, etc. ).
Many traditional pubs have closed. There is a drift into combined pub/restaurants. Not sure poker machines, as such are allowed, although we have other gaming machines in arcades/shops/tenpin bowling centres.