No, what I meant is that the roaming is completely transparent to the IP side of things, it happens at Layer 2 (or lower). The GGSN knows it’s from UAE because it’s receiving a connection (in the form of UDP packets, the protocol is GTP, or GPRS tunnelling protocol) from the SGSN based in UAE, however as far as the phone’s connection is concerned there is no difference - all of this is completely transparent to the phone and to its network side of things.
I guess you could have a GGSN (if you pay enough money to Ericsson, or are crazy enough to make your own) that assigns different IP pools based on where the actual tunnel comes from, but either way at that point the data has already arrived in the UK so no point of doing that, sending the data “back” to UAE (though you’d need something there to receive it, see next point) would be counter-productive and incur even more latency.
As far as I know there is no way to have a roaming SIM get an IP address from the roaming partner’s GGSN (so the data doesn’t have to go around the globe back to the UK) without explicit cooperation on their part, which is a shame but not surprising at all coming from such an industry.
You might understand better if you consider the GGSN as kind of a PPPoE server - for example on a DSL connection a trace route will show the first hop in your DSL provider’s equipment, despite having dozens of BT-owned routers and DSLAMs in the path as well - but those only forward PPP packets (the equivalent of GTP in the mobile world) and so are transparent to you.