Obviously, the pro is that it would give you more space. But, that comes at the price of making access and maintenance harder, etc. As long as the installation complies with the manufacture's installation instructions you can have it wherever you like.interesting that you don't like putting them in roofs. he seemed to think it would be better, suggesting it would give me more space (and he has a point; it's a very small flat!)
There's another point you should check first, however. If it's a flat, does your lease actually allow you to install a boiler in the roof? IME, it would be very unusual if that were the case. Do not assume that because the access point to the roof is in your flat that the roof space is included in your demised premises.