not with an old back boiler it wouldn't, but I would have thought it would be a high E / low D with the PV.Yes agree that doesn't add up. The rating should be more like C at least.
I'm assuming it must be detached, which also makes a fair amount of difference as it's got all 4 walls losing heat to the outside.
I'd think the best bet is going to be to persuade the customer to wait to submit the fit application until the boiler is sorted out, and see if they can bring that forward - renewable heat premium payments have just doubled which might help?