Samba Mobile is a mobile broadband provider that runs as a virtual network using the infrastructure of 3 Mobile. Because of this, it has very good coverage and decent download speeds. 3 is well-reviewed for its excellent 3G network and has won multiple awards from Ofcom about its mobile broadband signal.
You can check the Samba Mobile coverage map to find out about the signal strength in your local area. If you have particularly good or particularly bad in your area, please let others know here with a comment.