text: Ok, how about this for a least-worst option (changeset/183307500). Skip to summary below if this is too confusing.
  
  Given legislation (and signage) still says the A20 goes through the centre of town, I've left that ref in place - but reduced to Primary rather than Trunk. It now has the same status as the stretch of (still signposted) A21 leading into the centre from the south.
  
  On the ground, the circular section (Blair Mayne Road South, Messines Road, Castlebawn Road) is the only A20 route signposted in the eastbound direction, from the Kempe Stones Road roundabout (and signposted as trunk road, green signs), and it's also cited in Sabre. For a long time, the trunk route T11 has been tagged on this route, subsequently connecting with the A21 leading northeast of town. Given this routing, I've tagged way/60352036 as Trunk (despite its lack of green signage), and it remains as A20, connecting on to the aforementioned town centre section.
  
  The A21 coming from the south and leaving to the northeast is still severed (connected by the A20), but all sources suggest this is the case anyway.
  
  Summary: The only mapped "trunk" route is via the (signposted) town bypass. All else that is still signposted as A21 or A20 remains as such, but with primary tags.
  
  This is the most sense I can make out of the mess that actually exists. Does it work for you folks?