Does your loft have enough internal height to be converted to a room.

2400mm from the loft floor (plasterboard) to the highest part of the ridge?

If you do not have 2.4 Metres, then it's very unlikely!  Unless ceiling height of room's below proposed attic are high enough to be reduced to compensate.

In situations where a loft conversion does not need Planning Permission and the home owner opts not to apply for a Lawful Development Certificate from the Planning department, and where work does not effect a party wall requiring details to be submitted to a neighbouring home owner under the Party Wall Act, then a loft can be converted without plans. This involves completing a form and paying a fee to a Building Inspector who would oversee a builder who has enough knowledge to work without a plan.
However few home owners have enough building knowledge to ensure builders properly explain all the design options and once they have settled on a design, then verbally convey that design to a number of other builders with a view to obtaining like for like quotations. And when work starts ensure that the agreed product is received.
Identical homes will have loft conversions, however some of these lofts end up with better stair design and use of space plus more headroom than others.
Customers 'may' pay for 'independent' design advice and detailed Plans for a number of builders to quote, in which case the builders prices will be easier to compare, because all builders will be contracted to carry out identical work as per Plan.
An unfortunate scenario is where a home owner receives a loft conversion which turns out to be more basic than expected, and pays over the odds for that conversion, either because they did not have the plans to enable them to shop around, or because lack of detail resulted in them paying extra for what they were expecting within the original price.
To reiterate, Plans that do not have enough detail to maximize headroom etc, are of limited use.

Alternative to steels (which point load a building) is the aluminium design which spreads the load.