For your example you would edit..
RequiredHouses=British
For the unit like this...
[SNIPE]
RequiredHouses=British,French,Germans,Americans,Alliance
if you wanted even soviet and yuri sides to build this then make it like so...
[SNIPE]
RequiredHouses=British,French,Germans,Americans,Alliance,Russians,Confederation,Africans,Arabs,YuriCountry
Don't forget to edit the Owner part also so you don't get the New Construction options bug
[SNIPE]
RequiredHouses=British,French,Germans,Americans,Alliance,Russians,Confederation,Africans,Arabs,YuriCountry
Owner=British,French,Germans,Americans,Alliance,Russians,Confederation,Africans,Arabs,YuriCountry