I doubt a 757 flying into the UK is going to ask for a Flight Information Service.
Not so fast. A FIS is implicit in the service they receive as an IFR flight within controlled airspace and if they need something other than a clearance or position report, they've got exactly the same problem of getting the controllers attention without taking up too much airtime.
"Approach, Speedbird 123, request"
"Speedbird 123 standby"
[...]
"Speedbird 123, go ahead"
"Speedbird 123 request a weather update for XXX"
The "request" told the controller that something non-standard and potentially long-winding was forthcoming. But the exact same thing would be understood if the word "request" was left out.
And to make matters worse, amongst others Ryanair pilots are known to dial up INFO frequencies on box 2 to ask for weather updates if their main frequency is too busy and their destination is not on VOLMET.