Got okay speeds in Amsterdam. In France, it stayed on Orange, sometimes on LTE. This was in Paris.

Leaving Paris on Eurostar, it stayed Orange 3G throughout with good signal strength throughout, even in the tunnel sections.

Then it switched to O2 UK, some 3, some Vodaphone. This is in Chelsea. Saw some O2 on GPRS.

After not getting a local SIM in Amsterdam or Paris, I went and got an EE 4GEE sim for my iPad. SIM is free and the data bundle with 2 GB of LTE for 15 quid. There is also some voice and SMS, to be used within 30 days.