Last time I heard, prepaid data SIM cards were a thing, so I guess you could just get a second-hand one?
And lastly, especially if you live in west german you can just take a train to the netherlands and by a sim there, no registration required.
£25 for 12Gb of data would be ripoff here, unless it's roaming data.
EE [0] are one of the bigger carriers here, and the only ones with decent coverage where I live, this is their list of plans. You can get cheaper, but it often comes with limited speeds.