Triple8 has been treating me well with my latest FM13 project, I am connecting from Ohio to their host in DC and the speed seems comporable to closer hosts I've used. Triple8 has a west coast option too that might be closer for your purposes. I believe they have a trial so you can test it out.
Is distance a major problem? I'm looking for a decent and affordable FM13 host in the UK which is not an easy task. I had thought about a US based host as the UK ones seem slow on the uptake...
Ironically I have worked the opposite way (developing while connected to the UK) and the lag is noticeable compared to an inland server. Expect an additional 4-5 seconds any time the screen needs to redraw (IE scrolling, record paging, etc..)
I'd recommend checking with some of the filemaker consultants/platinum firms in the UK to see what they recommend for hosting.
(or maybe if you have some free time, rent a colo and start your own host!)
Have a look at our services:
We're in Raleigh, North Carolina. Not too far away, and we pay big bucks for very high quality bandwidth (23Gbps via 4 providers) because we know it counts with FileMaker.
Our shared FileMaker 13 hosting plans include FileMaker Go connections, but not WebDirect. WebDirect requires a virtual dedicated server with us due to the hardware requirements to make it work properly.
We also offer a 30 day trial on all our shared hosting accounts, so giving our services a try is risk free. Feel free to let me know if you have any additional questions either here or off-list to jmay(at)pointinspace.com.