I paid for the week long data pass when I was in the UK, after about 6 GB of transfer over a few days, the speed went back to around 0.6 Mbps. The included no charge roaming was around 0.12 Mbps.
This was not mentioned anywhere that there was a transfer limit on the data pass. After a week, I changed to the daily pass, cost more, but was more usable. The high latency wasn't good.
Next time I shall use a local SIM.