That's not a far less expensive plan per GB though.
20 GBP is about $25 USD. T Mobile US is offering 50GB LTE at $70 per month. Adjusting your 15GB to that nets you $83 per month. T Mobile also offers free call/text/data roaming as well to dozens of other countries as part of the plan.
Assuming Germany is on the list, that would still be a lot better than the 4GB cap + 5Eur per extra GB we have here. But to replace my landline I really need unlimited. Except, I don't really need to loose the landline other than to save for the expensive mobile contracts, and that only for streams.
