[inquisitor]

@bourbonkiller
Expect O2 to become AldiTalk's/eplus preferred - or maybe even only - roaming partner for the UK as Telefónica O2 Germany is in the process of acquiring eplus which will very likely result in all eplus subscribers (including all the MVNOs) to roam on Telefónica's networks at some point. So with an AldiTalk SIM you will perhaps have the same coverage as with toggle.
Unfortunately I didn't take notice which network I roamed on with my AldiTalk SIM last month when I was in London and used it all the day.

Be aware that you can overcome so-called steering of roaming by trying to register on an certain alternative network four times consecutively. You find out if a network is indeed blocked (no roaming agreement in place) or if you are just steered onto another network by the time your registration attempt takes. If you immediately receive a message like "network blocked", then it is indeed blocked, if it just takes ages to register and times out in the end, your home operator tries to steer you onto another network.