Possibly they are viable in the more tropical parts of GB such as London and the SE.
In Somerset in the hot summer of 2013 I grew 3 plants, all undercover, for a total harvest of a mere 2.6kg. I was not impressed, also in view of the whole season needed, and the cost of the slips.
Instead of sweet potatoes I love to grow Charlotte second early potatoes for double the yield and maturing in half the time, so I can grow another crop in the same season.
For you in London, the yield and time trade-off may be worth it if you value the flavour and nutrition sufficiently.
