As Said upthread the CTR-CRE Shuttles take just 19m to do 21m, but the Wrexham Shuttle which appears in December will be another contender, at around 15mins!
Do any Llandudno Jn - Town services run as Class 1s, i think a few of them have in the past!
Another contender for longest Class 2's are the Heart Of Wales Sprinters! From Crewe - Swansea
(I found a train in the May 2003 TT a while back that did Shrewsbury - Leamington Spa via the Heart of Wales, Swansea, Newport, Gloucester & Birmingham, surley one of the longest in recent history but that's not strictly relevant here!)