If you'll be using data for more than 8 days per month, Tchibo's monthly flatrate costing € 20 for 30 days is still the most competitive offer. Otherwise Fonic's daily flatrate (€ 2.50/day) with a price cap of € 25 per month is also an interesting option, especially since Fonic have very cheap international rates.
Details on both are available at
Germany - Prepaid Wireless Internet Access
Since data roaming is still exorbitantly expensive, you'll need to pick a separate SIM for the UK.
Regarding 3G modems, you should go for one from Huawei, as from my experience they are the best and easiest to handle, especially when it comes to
unlocking or
debranding. What you should consider is, that only few sticks support the major European 3G band (2100 MHz)
and AT&Ts 3G bands (850 & 1900 MHz). So if you want to use the device back in the Staates, your choice is limited to the Huawei E160 (available at some Tchibo stores, while others have already been restocked with the newer E1550, which unfortunately doesn't support American 3G bands like the E160
G) or the E1762 sold in Germany as
"Bild Mobil Speedstick" for € 50 (available only online), where the latter is much faster (7,2 MBit/s downstream vs. 3,6 MBit/s and 5,76 MBit/s upstream vs. 0,4 MBit/ts).