if you live in a terraced dwelling then you may only have two aspects to your roof.. if one is southerly then perfect..... if not then it may be an issue.. without knowing more then its hard to say...
as regards internal work to install solar panels... most systems require a new cylinder to be installed, which causes major work, however there is a system currently that doesnt need a new cylinder... see:
[broken link removed]
i havent personal experience with this, but it seems a good product.
From the wood pellet boiler point of view, id make the following comments:
if you dont have much rear garden space then bulk storage may be an issue... WPB are most economical when burning bulk pellets... buying in bag is currently as expensive a oil.
there seems to be an issue currently with the delivery of bulk pellets as well.. im open to contradiction on this.... especially in a terraced situation... something to do with the distance from truck to bulk storage..
oil burners start at about €700... WPB start at about €4000
is gas available to you?