Nice well-written post dude.
Anyway, I have to concur with some of the opinions here in that experimentation in the match against England is not the place to do it. I am well aware that SL look the better side, but in an ODI comp (as everyone knows) on the day anything can happen. Let us hope that SL does not have the hubris to tempt fate by not playing their best XI.
At this stage I would presume that XI would include Upul who despite having a poor run of late has done well against England in the past (however, I can also see a case for re-uniting the old firm of Sanath and Marvan at the top of the order and if that goes well then giving this pair an extended run at the top of the order). I do not think though, and here I have to disagree with Jason (as much as I usually respect his opinion) that Arnold should be dropped in place of Marvan. Arnold has done well with the bat when he has been asked to do so - both times against good opposition, he bats well with Dilshan, his bowling is an option, and he does well in the field. Therefore he should stay in.
In the same vein I do not think that Murali should be out in place of Bandara. The match against England is too important to rest Murali, and if any subbing is to happen, the best place to do it (albeit not wholesale) would be in the game against Ireland. Dilhara has, furthermore, been bowling quite well and though his batting is not in the same league as Maharoof - I think his bowling potency offsets the inconsistent batting of Maharoof.
Therefore my preferred XI against England (which I see as another must win because I think the chances of SL beating Aus and NZ are quite low) is: