If the MOD has a limited budget for advertising for recruitment then it would be better targeting it in areas where you are likely to get people to join up.
In an area where people hate the British Army then its not going to be the prime catchment area for getting recruits.
These communities were at war for years and so there's no point stirring things up. Yes we can say "you shouldn't be offended by this" but that just leads to stalemate, what happens is that starts bringing out belligerence amongst the dormant nationalist community there and you start hearing the old pledges of support for the IRA coming out.
I'm a Unionist on all issues about Scotland, NI etc and so I do regard this as Britain even in Catholic communities in NI where they don't necessarily like us, but I also take the view that these communities are our responsibility and we need to think of the potential benefits and potential costs of any action. If spending money on a recruitment billboard is going to cause anger and tensions thats a cost, is there a benefit that will outweigh it in terms of the number of recruits we would get? If no, then don't do it.
You could make the same argument for putting the recruitment billboards in mosques, no doubt that would generate some heated debate too - but if there are enough Muslims that want to join the RAF and fight for Britain then it might outweigh the costs. If not though, I wouldn't put the billboards there, I would put them in communities where you are likely to get a better return in terms of young recruits. There are loads of places in the UK where you could put them, where people would be positive about the armed forces, and you would get good keen young lads that want to serve their country, which is what you want for the RAF.