So, I'm guessing before I even think about a loft conversion the whole
roof
will need replacing to make it a higher pitched roof and the type of roof
that doesn't require the inward slanting pieces of wood for support.

Yes, as you have a detached house, it may be sensible to design a completely
new roof for it. The roof covering itself will cost not a million miles from
3000 quid. The cost of the rafters and roof structure that the covering sits
on I'm not so sure of, but a ****ing in the wind guess would be slightly
more than the same again, not including insulation etc. which come under the
general loft conversion costs anyway. However, I might be wrong on the cost
of the structure.

When rebuilding the roof, take advantage of the fact to use greater than the
required amount of insulation up there. You could even incorporate a air gap
and foil reflector before the insulation, which can help reduce heat gain in
summer, although using the space for real insulation instead might suffice,
if be a little more expensive.

Christian.