It's going to be your upload speed in UK.
I have my Slingcatcher in Spain, and have the luxury of two broadband connections (one for personal use, and one for business use) at my house in UK where my Slingbox is located.
My VirgingMedia Cable connection gives me streaming at around 1200 to 1300. My BT line usually gives me streaming at 250-350 (it occassionaly gets up to 600 - but that's extremely rare - usually during the silly hours of 4-5am).
BT are sh*te! Old and long telephone cables from their exchanges seriously limit bandwidth. VirginMedia Cable (not ADSL) connections are new and hi-tec.