It is not only UK airlines that use tactical alphanumeric callsigns; many airlines use them such as Lufthansa, Austrian and others.
I asked a Lufthansa pilot about this just yesterday when I flew through FRA and he explained that this was done to avoid misunderstandings between ATC and pilots when similar flight numbers from different airlines fly in the same busy ATC sector. So to avoid having to deal with Lufthansa 4350 and Speedbird 4350 at the same sector at the same time, these special seemingly random callsigns have been implemented (mainly in areas where these is much traffic).
As you say, for listeners this makes it hard to follow such flights, but one can get a database update at
www.acarsd.org that translates alpha callsigns to flight numbers.
Peter
this is very interesting, and absolutely different to how it is handled in the US. If there were similar callsigns under a given controller's control, ATC will advise them both of the similar callsign:
"United 732, be advised that a Frontier 732 is also on frequency."
"Frontier 732, be advised that a United 732 is also on frequency."
Then to further alleviate callsign confusion, ATC may repeat the company name after their callsign:
"United 732 United ...." "Frontier 732 Frontier ...."
Why change the callsign altogether, outside of if another flight with the same company callsign is in the air, is unusual.
BL.