I'm new here so some of these may already have been mentioned elsewhere. I'm rather partial to sausages and in Tesco I found:
Tesco's finest Hungarian Paprika Smoked Sausages. Not cheap but very tasty and 0 carbs/sausage.

Also, just tried their new smoked salmon and tuna sausages. Not fond of them but relatively LC at 1.4 carbs/sausage. (At 9g protein, approx 3g fat and 67 calories per sausage)

I found Splenda in Tesco and Boots at around £2.50.

Boots also had their chocolate for diabetics range. quite LC in Atkins terms in that most of the carbs are polyols but I know there's some controversy over their effects and also each bar (120g) had something like 3 times the RDA of polyols which is likely to have quite a laxative effect - so I stayed away from them.

Cheers