12v/18v diseqc motors are only for wall mount dishes up to about 1.2m in size, although some will claim up to 1.4m dish. If the dish is fairly protected and lightweight enough then a 1.4m offset should be possible.
Anything larger really becomes ground mounted as standard and wouldnt use a low voltage diseqc motor. You get 36volt actuator arms that slide in/out like an extending tube. Not all large ground mounted dishes are able to be motorised, they must have an actuator bracket (moveable structure) as part of its manufacture. For instance the cheap Fortec/maplin dishes 1.8m or 2.4m are not able to be motorised.
Additionally, there arent any new receivers that can provide 36volt actuator signals so you also need a positioner or a vbox to convert diseqc commands. It becomes expensive so not much point really.
Going off subject a bit, i have installed large amounts of 1.8m dishes to walls (not in uk) but none were motorised, although I did install a couple of 1.8m motorised fixed to flat roofs .
In the uk this type of dish will be hard to find as they arent popular enough, so you would likely have to import it from Europe.
1.8m Polar mount Dish will cost about 500 euros, plus transport cage about 350, plus delivery about 400-ish, and actuator/v-box cost at about 150 euros.
Some Dish examples here- _http://www.demon-multimedia.com/productos/productos.asp?ids=4
But not sure if they arrange delivery to uk?
I would expect allow about 1500 euros total should cover imported dish cost, actuator, positioner. You may need to do lots work to arrange though, unless you find a uk dealer but then you'd pay above 2000 euros.
I suppose you will maximising the signal from most satellites but for Show on 7W I think we need a bigger dish again to get all the channels? I guess it just depends how much you want to throw at this hobby really!!!
You probably wont find anymore channels than already on your 1.1m dish.
Your location is shown as Ireland so I doubt even a 2.4m will get you showtime anyway, and I wouldnt recommend large motorised to get Nilesat-H because fringe-reception~precision~motorised dont go together well.