[wolfbln]

What an advice for the bin from you:

no 4G/LTE: generally no 4G/LTE international roaming on Three UK SIM cards. This means a very limited coverage as 3G is not built up outside towns and cities in Germany. Here 3G coverage is only at around 60%, while 4G is at around 90% of population (not area) - And 3G means of course much slower speeds compared to 4G/LTE.
no hotspot data: no tethering allowed on UK Three PAYG SIM card. They sniff and kill tethering (mobile hotspot) connections through Deep Package Inspection
hard to top-up: no chance to top-up an Three UK SIM card without a UK registered credit card or you face high surcharges by some agencies. Recharge.com that you have linked charges around 10% fee for top-ups.

Why do you keep on suggesting these things?
-> without mentioning the strings attached?
When I consume a package of up to 12 GB what you have suggested, users need to have 4G/LTE available nowadays at least most of the time.
3G can still be sufficient for texting and messaging. But for these tasks, not 12 GB, rather 12 MB are needed.

I agree that UK SIM cards (not only Three) can be a good option for roaming in Germany.
Their big advantage is their easy activation which is very tricky in Germany right now through this online video chat.
I've checked all major UK providers, but they only offer a better rate on large packs (> 5GB) compared with usual German prepaid plans.
Three is the least appealing choice with the absence of 4G/LTE roaming and the top-up fees of the agencies.

All major UK operators (other than Three) offer 4G networks in Germany under "roam like at home" rules without surcharges at domestic UK prices:
EE on Telekom LTE and O2.DE LTE networks, but throttled - all credit cards accepted
Vodafone UK on Vodafone DE LTE network, unthrottled - all credit cards accepted
O2 UK and even giffgaff at least on O2 DE LTE network - giffgaff also accepts foreign credit cards