Sling might tell you that 2mbps is enough but in my experience with a Slingbox 350 is that you need to be able to stream at 3mbps in order to use the Best (HD) setting for a component source coming into the Slingbox at 1080i/25, such as Sky+ HD.
You won't get this with ADSL, the best speed you can get with ADSL is close to 2mbps via an 'Annex M' provider such as "Be", but more normally I think the uploads max out at a little over 1mbps with ADSL.
So you basically need Fibre, something like BT Infinity or equivalent, or Virgin Media. I get a solid 15mbps upload speed with BT Infinity which is more than enough.
Alternatively, consider renting a UK Slingbox - search on somewhere like placeshiftingenthusiasts website or Google 'Sling sharing' for other places where people offer rental services.
If you can live without HD then a 1-1.5mbps upload speed will get you a pretty solid SD picture which is fine for most viewing, although since this will only stream at 25fps it isn't great for watching sport. For watching sport you really need the 50fps that Best (HD) gives you in addition to its higher resolution.