You'll probably find connection to the UK server directly via a locally installed client is really only a workable solution over a LAN.
We have a TM1 server installed in London, with users distributed worldwide. Non-London users (until now - see below) have always connected to the London server via Citrix - this, or Terminal Services, seems to be the default deployment method for multi-site implementations where there is the desire to maintain a single, centralised server.
Off the top of my head, some of the benefits and pitfalls of Citrix are as follows;
BENEFITS
Can maintain a single instance of TM1 on one server (simplicity of TM1 implementation)
Easier to upgrade - only need to change client software on the Citrix machine (& server)
Ability to shadow users (great for helping users with functionality issues)
PITFALLS
Connection is vulnerable to latency or dropping out where there are bandwidth limitations
Needs configuring to allow users to see their usual network drives, use of printers etc
Takes forever for users to open a file stored on their local network onto the Citrix server (despite attempts to re-educate users in the use of application folders or storing files on the London network, a lot of them still struggle)
Where TM1 and/or Excel hang, it can be tricky for the user to kill their session and restart Excel - more often than not they need IT to intervene to kill citrix session
The extent to which the Pitfalls come into play will be dependent on the level of expertise in your IT group (and bandwidth)
ALTERNATIVES
REPLICATION - an alternative is to place an additional server(s) in other offices, then synchronise the data and structures on those servers on a scheduled (eg overnight) or adhoc basis as required.
This solution seems to get quite a slating on these forums - the interface for setting the functionality up is quite poor and useful documentation almost seems to be non-existant.
However, from my limited experience, I've found that if the replication is carefully thought out and implemented correctly, it can work quite well.
For my current client, we are piloting a replicated solution in the Aberdeen office and so far so good (touch wood!)
WEB - Maybe an option? The main downside in my experience has been that you miss out on most of the functionaility available in an excel-integrated solution. As you mention Sales offices below, the excel environment may not be so important for your target user?
Try a search on older topics on here as all of these approaches (inluding Citrix) have been discussed extensively before
