Don't know why the top british/english ballroom dancers are not in SCD. Agree with bendymixer that Anton and Erin are not anywhere near the top British couple in ballroom, and Anton (by his own admission) has little aptitude for latin, which is presumably why he and erin do not take part in the proffessional dance demonstration, which has been latin so far.
However, in the latin field, the top two British male dancers, Matthew Cutler and Darren Bennett (ranked no. 1 British couple with Lilia Kopylova) are in the SCD competition. So as far as latin goes, we do have the top British dancers. Also, in the UK closed competition, Ian and Camilla came 2nd to Darren and Lilia, so nationally they would rank highly too.
I assume they would not approach dancers representing other countries if they could get the top British couples/dancers.