Unfortunately Brano, in the late '60's / early '70's penny-pinching builders started adopting the "fink truss" roof, found in your house. This was invented in the 1940's during the War Effort to save decent timber for building aircraft etc. by reducing the strength (read size) of timber for roofing. There was no consideratioin given to the likelihood of anyone wanting to use the space in the loft, and other aspects of houses built this way are affected - the ceiling joists are designed only to support the plaster and loft insulation, for instance (water tanks often have to be sat on specially constructed platforms of "proper sturdy wood" to pass the loads to the walls), the upper wall structure is designed to take smaller dead loads as the fink truss is lighter by far than the traditional "cut roof" joists anad purlins. A by-product of this change is that roof pitches decreased from around 45 degrees to between 20 and 30 degrees, with a corresponding loss of loft height - to make an usable room one needs a height of at least 7' over most of the floor.
To even consider a loft conversion, you *need* the services of a structural engineer as the fink truss is an unitary structure - all the elements are required to share the loadings - and removing one element will quite likely mean that other elements are taken beyond their permitted loads, with the risk of collapse... it'll need a complete new roof strucure of full-sized timber, probaly steelwork as (for example) the spans betwen supports for the roof will exceed the building regulation defined maximum for unsupported purlins . While designing the new roof structure it''s likely that the architect / structural engineer will consider that the wall structure isn't adequate for the new loadings (particularly if there are stud-and-plaster internal walls or timber-frame external walls, requiring additional structural work down to foundation level... you'll need to install new floor joists alongside the existing ceiling joists (which can't be removed in advance, as they tie the building walls together and prevent the outward roof load toppling them), probably 8" by 2" minimum, raising the current "floor" level by 4" or so. The increase in "floor" height will reduce headroom, meaning you'l need to either raise the ridge line or install dormer windows... it goes on and on.
G/f and I are in the final stages of buying a house together, with the possibility (or probability) of converting the loft firmly in mind - we eliminated anything built since 1960 - 1970 for just these reasons!
My considered advice is: . . . Move house [1]. It'll be cheaper and less disruptive.
Dave H. (The engineer formerly known as Homeless)
[1] to a nice trad house made of bricks with a nice trad roof made of 8x2" and bigger timbers