Well actually they are collar ties to stop the roof from collapsing (which might also have some interesting affects on the walls if cut . . .) but as has been said these are not to be played with - and they are not easy to replace to offer the space you seem to want. As Pulse has said you should simply place some panelling on top of those ceiling joists and expect to put much load - or even be walking across the 'floor'. You could add some support beams parallel to that existing hanging beam and extending from wall to wall then place some flooring on top of those, but the ceiling structure was designed primarily to carry just the weight of the sub-structure itself and the ceiling lining (plaster) so you are probably restricted to maximum 200 kgs in total across the whole area (maybe less and that would include the weight of any 'flooring' you put in)- it was not designed to carry floor loads. Pity you didn't jump on here before you installed the ladder eh . . .
Originally Posted by seriph1
Advice from me on this forum is general and for guidance based on information given by the member posing the question. Not to be used in place of professional advice from people appropriately qualified in the relevant field. All structural work must be approved and constructed to the BCA or other relevant standards by suitably licensed persons. The person doing the work and reading my advice accepts responsibility for ensuring the work done accords with the applicable law.