Your Dapol 121 should be perfectly ok with a basic Hornby controller, I've just tested a Dapol Jinty on a yard of Peco track, using a 9v PP3 battery! Worked fine...
I'd suggest that a Gaugemaster Combi controller would be a better long term bet for the 121 and for Dapol 0-6-0 and similar locos. If you're intending to run anything bigger, a Gaugemaster GMC100-MO single controller or a GMC-DO double controller would be suitable. If you want to economise, the Morley Vanguard double controller mighr be a better bet.