Worcester Bosch, in my view, have the best post purchase support out there. Most AIs get a 5yr warranty.
I can see why WB would restrict their use in a loft. It is a big, heavy unit that will not pass up narrow stairs or through any normal loft hatch. In other words, a typical loft extension.
If you were unlucky enough to have a warranty issue that required boiler replacement, alterations made to the building after the boiler was installed could make it nearly impossible to remove.
WB could give chapter and verse in their MIs about access etc but builders would just fit the new stairs regardless after the boiler had been manhandled into position and it would be then effectively trapped forever.
I suspect that if your boiler was located in the 'main house' on the plans rather than in 'the loft', and you could show that there were no access pinch points, Worcester may treat it differently.
Please also bear in mind this boiler has a non removable internal condense pump which may wake you up if sited close to, or above a sleeping area.