It is difficult to choose an given Hornby model let alone engine so I would suggest some of the 0-6-0 tender locomotives that aren't immediately seen as popular mainstream models. For example the LNER J15 (if I could find an excuse for one of these I would) and the SR Q1.
Such superb models have the potential to appear on so many layouts; large or small and (in my humble opinion) are of greater modelling interest than (say) large mainline pacific locomotives of which so many types are produced.
The 2 BIL /2 HAL electric multiple units were also exciting models and it would be nice to see electric stock (both DC & AC) modelled from around the UK.
Clacton Mk1 sets or Glasgow Blue trains perhaps.....