Mmmm...
Nice thought. Lets build 4 new MR4A's, just for bombing...
NOT GONNA HAPPEN !
The MR4A's we have are revamped 50 year old airframes.
Yes, they are about 80 - 95% "New", but they really are what was the comet commercial aircraft from the 1950's.
The cost of building x4 additional aircraft would be prohibitive. In addition, the characteristics (usage, weight carrying capability, flight pattern/flying style required for bombing, maximum speed & number of hours flight life), would mean that although it seems like a good idea, the aircraft where never designed for this role & wouldn't real last that long.
Yes the MR2 / MR4A's have a bomb bay, but the full flight weight carrying capability is limited & is probably about the same as the maximum bomb load of 1 or 2 Tornado's.
For more info look here...
http://www.raf.mod.uk/equipment/nimrodmra4.cfm
http://www.raf.mod.uk/equipment/nimrodmr2.cfm
http://www.raf.mod.uk/equipment/nimrodr1.cfm
...& of course...
http://en.wikipedia.org/wiki/Hawker_Siddeley_Nimrod
http://en.wikipedia.org/wiki/De_Havilland_Comet
All that said, the comments already posted here pretty much elude to how the top brass see things. The US is there as an ally, to call on if we get involved in something large.
If we need to get something done by ourselves, it's probably gonna be small scale & could / will be covered by the Tornados & / or TLAM from subs.
The best aircraft we had in our arsenal is now gone. The Jaguar. Our American cousins relied heavily on us using it in the Gulf wars, as it's so different from most A/C they have & was so excellent for the role it filled.
Fighting tactics have evolved over the last 60 years (since WW II), we moved on from the carpet bombing style, as it's just not effective enough.
Wiping out a 1/2 km square of land / town / village, just to get at one factory / gun emplacement / army barracks, using 12 A/C with x10 - 2,000lb bombs isn't cost effective on resources of the airforce, especially when it can now be done with x1 A/C with x1 or x2 LGB's.
Society doesn't like the human costs either, never mind the financial / infrastructure / commercial costs to businesses that would be attacked / destroyed.
So take your pick. Stick with what we have, or get the Govt to modify an existing A/C that's not really up to the role, or fork out to design & build a brand new A/C for an almost outmoded form of warfare.