As you may (actually you probably aren't because of how obscure this was) be aware, the DUP
opened up a branch in Liverpool in 1999 and
sought recruits among the Orangemen of the city, with Glasgow and London cited as possible future expansions. I can't find exactly what happened, but I understand the failure of the Liverpool branch to be related to a lack of interest even among the cities Orangemen. Of course then in
2004 Basingstoke MP Andrew Hunter joined the DUP, even going so far as to, after leaving Parliament, make plans to start a new career in Ulster, though he couldn't in the end due to family issues. It's also notable that the DUP have had some mainland presence, such as their
infamous and surreal Metro wrap-around.
Now, it is notable that Hunter was a Monday Clubber, a member of the right-wing Conservative pressure group (though at the time of his defection the Monday Club had been deaffiliated for three years). It it also notable that the DUP wanted to exploit the vacuum left by the 1997 election in Liverpool. So what would be clear to me is that, for the DUP to really work on the mainland, it has to come in the wake of a far worse 1997 where there will be this far worse vacuum that can be exploited- perhaps even a genuine split in the Conservative Party. There is some appeal in working class oriented social conservatism and British Unionism, however the question becomes in how much initial support the DUP!Mainland can garner (be it council defections and MP's joining), and how far they can expand before the Conservative Party stabilises.