My phone (SE T630) has options for GPRS header compression. When I've
experimented with the settings, the data counters in the phone show the
same amount of transfer.

Am I actually compressing the IP header? Do the UK networks differ on
how header compression is provided? And am I charged per 'uncompressed'
byte, or only the bytes sent over the air interface?

Apart from a (potential?) cost-saving, presumably header compression
also give a small improvement in latency. And also increases slightly
the capacity of a GPRS cell. As IPv6 is introduced, and more real-time
(i.e. small payload) services are carried over GPRS, will header
compression necessary?

(I am aware than all operators are providing data compression
on-the-fly for web pages etc. in their proxy servers - that's not what
I'm referring to).

regards,
Andrew