You'll have to look back at your offer for the T&Cs as there were several different offers made. I think most of the 25K ones applied to a range of fares (including rather unusually some discount economy like R and M in addition to JCDIYBH). IIRC for the 50K ones, they were a lot tighter such that even I class was not included.

Check if I class is valid for the offer you were sent and then you can look at some of the cheapie ex-LGW trips like DUB if it is.

And yes, UK Domestic counts (provided the ticket is in the right class).

Phil