Nice to see some photos. Your roof looks to be covered with Redland 49 interlocking concrete tiles. These look ok, but really should have an underlay, which quite rightly as you suggest has perished. The danger here is that a defect in the tiles will allow water direct access through the covering. I also note some daylight through some of the tiles, suggest that water can also get in! This is an issue that really needs to be attended to first.
Your second point refers to using the loft. Word of caution: loft spaces are desgined for a light storeage load (typically 0.25kN/sqm). A domestic live load (ie bedroom, livign room etc) is 1.5kN/sqm, a six times increase so the loft space should not be used other than for light storage.
Hope this helps.