A SP switched device would actually satisfy BS 7671, other than in a TT installation. Although it's normal practice to use DP switching for isolation, Reg. 460-01-04 states that the neutral does not need to be isolated in a TN-S or TN-C-S earthed installation fed from the public mains. The exception is the main switch of a domestic installation, which must always be DP [476-01-03]. See also page 34 of the OSG.